--- a/browser/base/content/aboutDialog-appUpdater-legacy.js
+++ b/browser/base/content/aboutDialog-appUpdater-legacy.js
@@ -1,13 +1,13 @@
 /* This Source Code Form is subject to the terms of the Mozilla Public
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
-// Note: this file is included in aboutDialog.xul and preferences/advanced.xhtml
+// Note: this file is included in aboutDialog.xhtml and preferences/advanced.xhtml
 // if MOZ_UPDATER is defined.
 
 /* import-globals-from aboutDialog.js */
 
 const PREF_APP_UPDATE_CANCELATIONS_OSX = "app.update.cancelations.osx";
 const PREF_APP_UPDATE_ELEVATE_NEVER = "app.update.elevate.never";
 
 function onUnload(aEvent) {

--- a/browser/base/content/aboutDialog-appUpdater.js
+++ b/browser/base/content/aboutDialog-appUpdater.js
@@ -1,13 +1,13 @@
 /* This Source Code Form is subject to the terms of the Mozilla Public
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
-// Note: this file is included in aboutDialog.xul and preferences/advanced.xhtml
+// Note: this file is included in aboutDialog.xhtml and preferences/advanced.xhtml
 // if MOZ_UPDATER is defined.
 
 /* import-globals-from aboutDialog.js */
 
 // These two eslint directives should be removed when we remove handling for the
 // legacy app updater.
 /* eslint-disable prettier/prettier */
 /* global AppUpdater, appUpdater, onUnload */

rename from browser/base/content/aboutDialog.xul
rename to browser/base/content/aboutDialog.xhtml

--- a/browser/base/content/browser-places.js
+++ b/browser/base/content/browser-places.js
@@ -603,17 +603,17 @@ var PlacesCommandHook = {
    *                      AllBookmarks, History, Downloads.
    */
   showPlacesOrganizer(item) {
     var organizer = Services.wm.getMostRecentWindow("Places:Organizer");
     // Due to bug 528706, getMostRecentWindow can return closed windows.
     if (!organizer || organizer.closed) {
       // No currently open places window, so open one with the specified mode.
       openDialog(
-        "chrome://browser/content/places/places.xul",
+        "chrome://browser/content/places/places.xhtml",
         "",
         "chrome,toolbar=yes,dialog=no,resizable",
         item
       );
     } else {
       organizer.PlacesOrganizer.selectLeftPaneContainerByHierarchy(item);
       organizer.focus();
     }

--- a/browser/base/content/browser-sidebar.js
+++ b/browser/base/content/browser-sidebar.js
@@ -12,28 +12,28 @@ var SidebarUI = {
     }
     return (this._sidebars = new Map([
       [
         "viewBookmarksSidebar",
         {
           title: document
             .getElementById("sidebar-switcher-bookmarks")
             .getAttribute("label"),
-          url: "chrome://browser/content/places/bookmarksSidebar.xul",
+          url: "chrome://browser/content/places/bookmarksSidebar.xhtml",
           menuId: "menu_bookmarksSidebar",
           buttonId: "sidebar-switcher-bookmarks",
         },
       ],
       [
         "viewHistorySidebar",
         {
           title: document
             .getElementById("sidebar-switcher-history")
             .getAttribute("label"),
-          url: "chrome://browser/content/places/historySidebar.xul",
+          url: "chrome://browser/content/places/historySidebar.xhtml",
           menuId: "menu_historySidebar",
           buttonId: "sidebar-switcher-history",
           triggerButtonId: "appMenuViewHistorySidebar",
         },
       ],
       [
         "viewTabsSidebar",
         {

--- a/browser/base/content/browser-sync.js
+++ b/browser/base/content/browser-sync.js
@@ -163,17 +163,17 @@ var gSync = {
 
     // Label for the sync buttons.
     if (!this.appMenuLabel) {
       // We are in a window without our elements - just abort now, without
       // setting this._initialized, so we don't attempt to remove observers.
       return;
     }
     // We start with every menuitem hidden (except for the "setup sync" state),
-    // so that we don't need to init the sync UI on windows like pageInfo.xul
+    // so that we don't need to init the sync UI on windows like pageInfo.xhtml
     // (see bug 1384856).
     // maybeUpdateUIState() also optimizes for this - if we should be in the
     // "setup sync" state, that function assumes we are already in it and
     // doesn't re-initialize the UI elements.
     document.getElementById("sync-setup").hidden = false;
     document.getElementById("PanelUI-remotetabs-setupsync").hidden = false;
 
     for (let topic of this._obs) {

--- a/browser/base/content/browser.js
+++ b/browser/base/content/browser.js
@@ -3235,17 +3235,17 @@ function BrowserPageInfo(
       currentWindow.focus();
       currentWindow.resetPageInfo(args);
       return currentWindow;
     }
   }
 
   // We didn't find a matching window, so open a new one.
   return openDialog(
-    "chrome://browser/content/pageinfo/pageInfo.xul",
+    "chrome://browser/content/pageinfo/pageInfo.xhtml",
     "",
     "chrome,toolbar,dialog=no,resizable",
     args
   );
 }
 
 /**
  * Sets the URI to display in the location bar.

--- a/browser/base/content/browser.xhtml
+++ b/browser/base/content/browser.xhtml
@@ -689,17 +689,17 @@
 #endif
                context="toolbar-context-menu">
         <toolbaritem id="menubar-items" align="center">
 # The entire main menubar is placed into browser-menubar.inc, so that it can be
 # shared with other top level windows in macWindow.inc.xhtml.
 #include browser-menubar.inc
         </toolbaritem>
         <spacer flex="1" skipintoolbarset="true" ordinal="1000"/>
-#include titlebar-items.inc.xul
+#include titlebar-items.inc.xhtml
       </toolbar>
 
       <toolbar id="TabsToolbar"
                class="browser-toolbar titlebar-color"
                fullscreentoolbar="true"
                customizable="true"
                customizationtarget="TabsToolbar-customization-target"
                mode="icons"
@@ -758,17 +758,17 @@
         <hbox class="titlebar-spacer" type="post-tabs"/>
 
 #ifndef XP_MACOSX
         <button class="accessibility-indicator" tooltiptext="&accessibilityIndicator.tooltip;"
                 aria-live="polite"/>
         <hbox class="private-browsing-indicator"/>
 #endif
 
-#include titlebar-items.inc.xul
+#include titlebar-items.inc.xhtml
 
 #ifdef XP_MACOSX
         <!-- OS X does not natively support RTL for its titlebar items, so we prevent this secondary
              buttonbox from reversing order in RTL by forcing an LTR direction. -->
         <hbox id="titlebar-secondary-buttonbox" dir="ltr">
           <button class="accessibility-indicator" tooltiptext="&accessibilityIndicator.tooltip;" aria-live="polite"/>
           <hbox class="private-browsing-indicator"/>
           <hbox id="titlebar-fullscreen-button"/>

rename from browser/base/content/newInstall.xul
rename to browser/base/content/newInstall.xhtml

--- a/browser/base/content/nsContextMenu.js
+++ b/browser/base/content/nsContextMenu.js
@@ -1327,17 +1327,17 @@ class nsContextMenu {
         let image = document.createElementNS(
           "http://www.w3.org/1999/xhtml",
           "img"
         );
         image.src = dataURL;
 
         // Confirm since it's annoying if you hit this accidentally.
         const kDesktopBackgroundURL =
-          "chrome://browser/content/setDesktopBackground.xul";
+          "chrome://browser/content/setDesktopBackground.xhtml";
 
         if (AppConstants.platform == "macosx") {
           // On Mac, the Set Desktop Background window is not modal.
           // Don't open more than one Set Desktop Background window.
           let dbWin = Services.wm.getMostRecentWindow(
             "Shell:SetDesktopBackground"
           );
           if (dbWin) {

rename from browser/base/content/pageinfo/pageInfo.xul
rename to browser/base/content/pageinfo/pageInfo.xhtml

rename from browser/base/content/safeMode.xul
rename to browser/base/content/safeMode.xhtml

rename from browser/base/content/sanitize.xul
rename to browser/base/content/sanitize.xhtml

--- a/browser/base/content/test/sanitize/browser_sanitizeDialog.js
+++ b/browser/base/content/test/sanitize/browser_sanitizeDialog.js
@@ -773,17 +773,17 @@ WindowHelper.prototype = {
       // Retrieve the browser window so we can specify it as the parent
       // of the dialog to simulate the way the user opens the dialog
       // on Windows and Linux.
       browserWin = Services.wm.getMostRecentWindow("navigator:browser");
     }
 
     Services.ww.openWindow(
       browserWin,
-      "chrome://browser/content/sanitize.xul",
+      "chrome://browser/content/sanitize.xhtml",
       "SanitizeDialog",
       "chrome,titlebar,dialog,centerscreen,modal",
       null
     );
   },
 
   /**
    * Selects a duration in the duration dropdown.

--- a/browser/base/content/test/webrtc/head.js
+++ b/browser/base/content/test/webrtc/head.js
@@ -44,17 +44,18 @@ function promiseIndicatorWindow() {
   }
 
   return new Promise(resolve => {
     Services.obs.addObserver(function obs(win) {
       win.addEventListener(
         "load",
         function() {
           if (
-            win.location.href !== "chrome://browser/content/webrtcIndicator.xul"
+            win.location.href !==
+            "chrome://browser/content/webrtcIndicator.xhtml"
           ) {
             info("ignoring a window with this url: " + win.location.href);
             return;
           }
 
           Services.obs.removeObserver(obs, "domwindowopened");
           executeSoon(() => resolve(win));
         },

rename from browser/base/content/titlebar-items.inc.xul
rename to browser/base/content/titlebar-items.inc.xhtml

--- a/browser/base/content/utilityOverlay.js
+++ b/browser/base/content/utilityOverlay.js
@@ -986,17 +986,17 @@ function openAboutDialog() {
   if (AppConstants.platform == "win") {
     features += "centerscreen,dependent";
   } else if (AppConstants.platform == "macosx") {
     features += "resizable=no,minimizable=no";
   } else {
     features += "centerscreen,dependent,dialog=no";
   }
 
-  window.openDialog("chrome://browser/content/aboutDialog.xul", "", features);
+  window.openDialog("chrome://browser/content/aboutDialog.xhtml", "", features);
 }
 
 function openPreferences(paneID, extraArgs) {
   // This function is duplicated from preferences.js.
   function internalPrefCategoryNameToFriendlyName(aName) {
     return (aName || "").replace(/^pane./, function(toReplace) {
       return toReplace[4].toLowerCase();
     });

--- a/browser/base/content/webext-panels.js
+++ b/browser/base/content/webext-panels.js
@@ -1,14 +1,14 @@
 /* -*- indent-tabs-mode: nil; js-indent-level: 4 -*- */
 /* This Source Code Form is subject to the terms of the Mozilla Public
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
-// Via webext-panels.xul
+// Via webext-panels.xhtml
 /* import-globals-from browser.js */
 /* import-globals-from nsContextMenu.js */
 
 ChromeUtils.defineModuleGetter(
   this,
   "ExtensionParent",
   "resource://gre/modules/ExtensionParent.jsm"
 );

rename from browser/base/content/webext-panels.xul
rename to browser/base/content/webext-panels.xhtml

rename from browser/base/content/webrtcIndicator.xul
rename to browser/base/content/webrtcIndicator.xhtml

--- a/browser/base/jar.mn
+++ b/browser/base/jar.mn
@@ -1,17 +1,17 @@
 # This Source Code Form is subject to the terms of the Mozilla Public
 # License, v. 2.0. If a copy of the MPL was not distributed with this
 # file, You can obtain one at http://mozilla.org/MPL/2.0/.
 browser.jar:
 %  content browser %content/browser/ contentaccessible=yes
 
         content/browser/aboutDialog-appUpdater.js     (content/aboutDialog-appUpdater.js)
         content/browser/aboutDialog-appUpdater-legacy.js     (content/aboutDialog-appUpdater-legacy.js)
-*       content/browser/aboutDialog.xul               (content/aboutDialog.xul)
+*       content/browser/aboutDialog.xhtml                    (content/aboutDialog.xhtml)
         content/browser/aboutDialog.js                (content/aboutDialog.js)
         content/browser/aboutDialog.css               (content/aboutDialog.css)
         content/browser/aboutRestartRequired.js       (content/aboutRestartRequired.js)
         content/browser/aboutRestartRequired.xhtml    (content/aboutRestartRequired.xhtml)
         content/browser/aboutRobots.xhtml             (content/aboutRobots.xhtml)
         content/browser/aboutRobots.js                (content/aboutRobots.js)
         content/browser/aboutRobots.css               (content/aboutRobots.css)
 
@@ -74,56 +74,56 @@ browser.jar:
         content/browser/defaultthemes/3.preview.png   (content/defaultthemes/3.preview.png)
         content/browser/defaultthemes/4.header.png    (content/defaultthemes/4.header.png)
         content/browser/defaultthemes/4.icon.png      (content/defaultthemes/4.icon.png)
         content/browser/defaultthemes/4.preview.png   (content/defaultthemes/4.preview.png)
         content/browser/defaultthemes/5.header.png    (content/defaultthemes/5.header.png)
         content/browser/defaultthemes/5.icon.jpg      (content/defaultthemes/5.icon.jpg)
         content/browser/defaultthemes/5.preview.jpg   (content/defaultthemes/5.preview.jpg)
         content/browser/history-swipe-arrow.svg       (content/history-swipe-arrow.svg)
-*       content/browser/pageinfo/pageInfo.xul         (content/pageinfo/pageInfo.xul)
+*       content/browser/pageinfo/pageInfo.xhtml       (content/pageinfo/pageInfo.xhtml)
         content/browser/pageinfo/pageInfo.js          (content/pageinfo/pageInfo.js)
         content/browser/pageinfo/pageInfo.css         (content/pageinfo/pageInfo.css)
         content/browser/pageinfo/permissions.js       (content/pageinfo/permissions.js)
         content/browser/pageinfo/security.js          (content/pageinfo/security.js)
         content/browser/content-refreshblocker.js     (content/content-refreshblocker.js)
         content/browser/robot.ico                     (content/robot.ico)
         content/browser/static-robot.png              (content/static-robot.png)
         content/browser/safeMode.css                  (content/safeMode.css)
         content/browser/safeMode.js                   (content/safeMode.js)
-        content/browser/safeMode.xul                  (content/safeMode.xul)
-        content/browser/sanitize.xul                  (content/sanitize.xul)
+        content/browser/safeMode.xhtml                (content/safeMode.xhtml)
+        content/browser/sanitize.xhtml                (content/sanitize.xhtml)
         content/browser/sanitizeDialog.js             (content/sanitizeDialog.js)
         content/browser/sanitizeDialog.css            (content/sanitizeDialog.css)
         content/browser/contentSearchUI.js            (content/contentSearchUI.js)
         content/browser/contentSearchUI.css           (content/contentSearchUI.css)
         content/browser/tabbrowser.css                (content/tabbrowser.css)
         content/browser/tabbrowser.js                 (content/tabbrowser.js)
         content/browser/tabbrowser-tab.js             (content/tabbrowser-tab.js)
         content/browser/tabbrowser-tabs.js            (content/tabbrowser-tabs.js)
         content/browser/utilityOverlay.js             (content/utilityOverlay.js)
         content/browser/webext-panels.js              (content/webext-panels.js)
-*       content/browser/webext-panels.xul             (content/webext-panels.xul)
+*       content/browser/webext-panels.xhtml           (content/webext-panels.xhtml)
         content/browser/nsContextMenu.js              (content/nsContextMenu.js)
         content/browser/contentTheme.js               (content/contentTheme.js)
 #ifdef XP_MACOSX
 # XXX: We should exclude this one as well (bug 71895)
 *       content/browser/hiddenWindowMac.xhtml         (content/hiddenWindowMac.xhtml)
         content/browser/nonbrowser-mac.js             (content/nonbrowser-mac.js)
 #endif
 #ifndef XP_MACOSX
-*       content/browser/webrtcIndicator.xul           (content/webrtcIndicator.xul)
+*       content/browser/webrtcIndicator.xhtml           (content/webrtcIndicator.xhtml)
         content/browser/webrtcIndicator.js            (content/webrtcIndicator.js)
 #endif
 # the following files are browser-specific overrides
 *       content/browser/license.html                  (/toolkit/content/license.html)
 % override chrome://global/content/license.html chrome://browser/content/license.html
         content/browser/blockedSite.xhtml               (content/blockedSite.xhtml)
         content/browser/blockedSite.js                  (content/blockedSite.js)
-        content/browser/newInstall.xul                (content/newInstall.xul)
+        content/browser/newInstall.xhtml                (content/newInstall.xhtml)
         content/browser/newInstall.js                 (content/newInstall.js)
         content/browser/newInstallPage.html           (content/newInstallPage.html)
         content/browser/newInstallPage.js             (content/newInstallPage.js)
 
 % override chrome://global/content/netError.xhtml chrome://browser/content/aboutNetError.xhtml
 
 # L10n resources and overrides.
 % override chrome://global/locale/appstrings.properties chrome://browser/locale/appstrings.properties

--- a/browser/components/BrowserGlue.jsm
+++ b/browser/components/BrowserGlue.jsm
@@ -160,18 +160,18 @@ let ACTORS = {
     },
     includeChrome: true,
     allFrames: true,
     matches: [
       "about:home",
       "about:newtab",
       "about:welcome",
       "chrome://browser/content/syncedtabs/sidebar.xhtml",
-      "chrome://browser/content/places/historySidebar.xul",
-      "chrome://browser/content/places/bookmarksSidebar.xul",
+      "chrome://browser/content/places/historySidebar.xhtml",
+      "chrome://browser/content/places/bookmarksSidebar.xhtml",
     ],
   },
 
   LinkHandler: {
     parent: {
       moduleURI: "resource:///actors/LinkHandlerParent.jsm",
     },
     child: {
@@ -1142,17 +1142,17 @@ BrowserGlue.prototype = {
     } else {
       PdfJs.earlyInit();
     }
 
     // check if we're in safe mode
     if (Services.appinfo.inSafeMode) {
       Services.ww.openWindow(
         null,
-        "chrome://browser/content/safeMode.xul",
+        "chrome://browser/content/safeMode.xhtml",
         "_blank",
         "chrome,centerscreen,modal,resizable=no",
         null
       );
     }
 
     // apply distribution customizations
     this._distributionCustomizer.applyCustomizations();
@@ -1913,17 +1913,17 @@ BrowserGlue.prototype = {
 
       let stack = win.gBrowser.getPanel().querySelector(".browserStack");
       let mask = win.document.createElementNS(XULNS, "box");
       mask.setAttribute("id", "content-mask");
       stack.appendChild(mask);
 
       Services.ww.openWindow(
         win,
-        "chrome://browser/content/newInstall.xul",
+        "chrome://browser/content/newInstall.xhtml",
         "_blank",
         "chrome,modal,resizable=no,centerscreen",
         null
       );
       mask.remove();
     });
   },
 

--- a/browser/components/about/AboutRedirector.cpp
+++ b/browser/components/about/AboutRedirector.cpp
@@ -104,17 +104,17 @@ static const RedirEntry kRedirMap[] = {
     {"pocket-signup", "chrome://pocket/content/panels/signup.html",
      nsIAboutModule::URI_SAFE_FOR_UNTRUSTED_CONTENT |
          nsIAboutModule::URI_CAN_LOAD_IN_CHILD | nsIAboutModule::ALLOW_SCRIPT |
          nsIAboutModule::HIDE_FROM_ABOUTABOUT},
     {"preferences",
      "chrome://browser/content/preferences/in-content/preferences.xhtml",
      nsIAboutModule::ALLOW_SCRIPT},
     {"downloads",
-     "chrome://browser/content/downloads/contentAreaDownloadsView.xul",
+     "chrome://browser/content/downloads/contentAreaDownloadsView.xhtml",
      nsIAboutModule::ALLOW_SCRIPT},
     {"reader", "chrome://global/content/reader/aboutReader.html",
      nsIAboutModule::URI_SAFE_FOR_UNTRUSTED_CONTENT |
          nsIAboutModule::ALLOW_SCRIPT | nsIAboutModule::URI_MUST_LOAD_IN_CHILD |
          nsIAboutModule::HIDE_FROM_ABOUTABOUT},
     {"restartrequired", "chrome://browser/content/aboutRestartRequired.xhtml",
      nsIAboutModule::ALLOW_SCRIPT | nsIAboutModule::HIDE_FROM_ABOUTABOUT},
     {"newinstall", "chrome://browser/content/newInstallPage.html",

rename from browser/components/downloads/content/contentAreaDownloadsView.xul
rename to browser/components/downloads/content/contentAreaDownloadsView.xhtml

--- a/browser/components/downloads/jar.mn
+++ b/browser/components/downloads/jar.mn
@@ -2,12 +2,12 @@
 # License, v. 2.0. If a copy of the MPL was not distributed with this
 # file, You can obtain one at http://mozilla.org/MPL/2.0/.
 
 browser.jar:
         content/browser/downloads/downloads.css          (content/downloads.css)
         content/browser/downloads/downloads.js           (content/downloads.js)
         content/browser/downloads/indicator.js           (content/indicator.js)
         content/browser/downloads/allDownloadsView.js    (content/allDownloadsView.js)
-*       content/browser/downloads/contentAreaDownloadsView.xul (content/contentAreaDownloadsView.xul)
+*       content/browser/downloads/contentAreaDownloadsView.xhtml (content/contentAreaDownloadsView.xhtml)
         content/browser/downloads/contentAreaDownloadsView.js  (content/contentAreaDownloadsView.js)
         content/browser/downloads/contentAreaDownloadsView.css (content/contentAreaDownloadsView.css)
         content/browser/downloads/downloadsCommands.js         (content/downloadsCommands.js)

--- a/browser/components/downloads/test/browser/head.js
+++ b/browser/components/downloads/test/browser/head.js
@@ -179,17 +179,17 @@ function startServer() {
 function httpUrl(aFileName) {
   return (
     "http://localhost:" + gHttpServer.identity.primaryPort + "/" + aFileName
   );
 }
 
 function openLibrary(aLeftPaneRoot) {
   let library = window.openDialog(
-    "chrome://browser/content/places/places.xul",
+    "chrome://browser/content/places/places.xhtml",
     "",
     "chrome,toolbar=yes,dialog=no,resizable",
     aLeftPaneRoot
   );
 
   return new Promise(resolve => {
     waitForFocus(resolve, library);
   });

--- a/browser/components/enterprisepolicies/tests/browser/browser_policy_disable_profile_import.js
+++ b/browser/components/enterprisepolicies/tests/browser/browser_policy_disable_profile_import.js
@@ -1,16 +1,16 @@
 /* Any copyright is dedicated to the Public Domain.
  * http://creativecommons.org/publicdomain/zero/1.0/ */
 "use strict";
 
 async function openLibrary() {
   return new Promise(resolve => {
     let library = window.openDialog(
-      "chrome://browser/content/places/places.xul",
+      "chrome://browser/content/places/places.xhtml",
       "",
       "chrome,toolbar=yes,dialog=no,resizable"
     );
     waitForFocus(() => resolve(library), library);
   });
 }
 
 add_task(async function test_disable_profile_import() {

--- a/browser/components/enterprisepolicies/tests/browser/disable_app_update/browser_policy_disable_app_update.js
+++ b/browser/components/enterprisepolicies/tests/browser/disable_app_update/browser_policy_disable_app_update.js
@@ -67,17 +67,17 @@ add_task(async function test_update_abou
 function waitForAboutDialog() {
   return new Promise(resolve => {
     var listener = {
       onOpenWindow: aXULWindow => {
         Services.wm.removeListener(listener);
 
         async function aboutDialogOnLoad() {
           domwindow.removeEventListener("load", aboutDialogOnLoad, true);
-          let chromeURI = "chrome://browser/content/aboutDialog.xul";
+          let chromeURI = "chrome://browser/content/aboutDialog.xhtml";
           is(
             domwindow.document.location.href,
             chromeURI,
             "About dialog appeared"
           );
           resolve(domwindow);
         }
 

--- a/browser/components/extensions/parent/ext-devtools-panels.js
+++ b/browser/components/extensions/parent/ext-devtools-panels.js
@@ -15,17 +15,17 @@ ChromeUtils.defineModuleGetter(
   "BroadcastConduit",
   "resource://gre/modules/ConduitsParent.jsm"
 );
 
 var { IconDetails, watchExtensionProxyContextLoad } = ExtensionParent;
 
 var { promiseDocumentLoaded } = ExtensionUtils;
 
-const WEBEXT_PANELS_URL = "chrome://browser/content/webext-panels.xul";
+const WEBEXT_PANELS_URL = "chrome://browser/content/webext-panels.xhtml";
 
 class BaseDevToolsPanel {
   constructor(context, panelOptions) {
     const toolbox = context.devToolsToolbox;
     if (!toolbox) {
       // This should never happen when this constructor is called with a valid
       // devtools extension context.
       throw Error("Missing mandatory toolbox");
@@ -442,17 +442,17 @@ class ParentDevToolsInspectorSidebar ext
     if (this.containerEl && this.panelOptions.url) {
       this.createBrowserElement(this.containerEl.contentWindow);
     }
   }
 
   onExtensionPageMount(containerEl) {
     this.containerEl = containerEl;
 
-    // Wait the webext-panel.xul page to have been loaded in the
+    // Wait the webext-panel.xhtml page to have been loaded in the
     // inspector sidebar panel.
     promiseDocumentLoaded(containerEl.contentDocument).then(() => {
       this.createBrowserElement(containerEl.contentWindow);
     });
   }
 
   onExtensionPageUnmount() {
     this.containerEl = null;

--- a/browser/components/extensions/parent/ext-sidebarAction.js
+++ b/browser/components/extensions/parent/ext-sidebarAction.js
@@ -12,17 +12,17 @@ var { ExtensionParent } = ChromeUtils.im
 
 var { ExtensionError } = ExtensionUtils;
 
 var { IconDetails } = ExtensionParent;
 
 // WeakMap[Extension -> SidebarAction]
 let sidebarActionMap = new WeakMap();
 
-const sidebarURL = "chrome://browser/content/webext-panels.xul";
+const sidebarURL = "chrome://browser/content/webext-panels.xhtml";
 
 /**
  * Responsible for the sidebar_action section of the manifest as well
  * as the associated sidebar browser.
  */
 this.sidebarAction = class extends ExtensionAPI {
   static for(extension) {
     return sidebarActionMap.get(extension);

--- a/browser/components/migration/MigrationUtils.jsm
+++ b/browser/components/migration/MigrationUtils.jsm
@@ -991,17 +991,17 @@ var MigrationUtils = Object.freeze({
         params.appendElement(comtaminatedVal);
       }
     } else {
       params = aParams;
     }
 
     Services.ww.openWindow(
       aOpener,
-      "chrome://browser/content/migration/migration.xul",
+      "chrome://browser/content/migration/migration.xhtml",
       "_blank",
       features,
       params
     );
   },
 
   /**
    * Show the migration wizard for startup-migration.  This should only be
@@ -1013,17 +1013,17 @@ var MigrationUtils = Object.freeze({
    *        the nsIProfileStartup instance provided to ProfileMigrator.migrate.
    * @param [optional] aMigratorKey
    *        If set, the migration wizard will import from the corresponding
    *        migrator, bypassing the source-selection page.  Otherwise, the
    *        source-selection page will be displayed, either with the default
    *        browser selected, if it could be detected and if there is a
    *        migrator for it, or with the first option selected as a fallback
    *        (The first option is hardcoded to be the most common browser for
-   *         the OS we run on.  See migration.xul).
+   *         the OS we run on.  See migration.xhtml).
    * @param [optional] aProfileToMigrate
    *        If set, the migration wizard will import from the profile indicated.
    * @throws if aMigratorKey is invalid or if it points to a non-existent
    *         source.
    */
   startupMigration: function MU_startupMigrator(
     aProfileStartup,
     aMigratorKey,

rename from browser/components/migration/content/migration.xul
rename to browser/components/migration/content/migration.xhtml

--- a/browser/components/migration/jar.mn
+++ b/browser/components/migration/jar.mn
@@ -1,8 +1,8 @@
 # This Source Code Form is subject to the terms of the Mozilla Public
 # License, v. 2.0. If a copy of the MPL was not distributed with this
 # file, You can obtain one at http://mozilla.org/MPL/2.0/.
 
 browser.jar:
-*  content/browser/migration/migration.xul                    (content/migration.xul)
+*  content/browser/migration/migration.xhtml                  (content/migration.xhtml)
    content/browser/migration/migration.js                     (content/migration.js)
    content/browser/aboutWelcomeBack.xhtml                     (content/aboutWelcomeBack.xhtml)

--- a/browser/components/newtab/test/xpcshell/test_AboutNewTabService.js
+++ b/browser/components/newtab/test/xpcshell/test_AboutNewTabService.js
@@ -16,17 +16,17 @@ XPCOMUtils.defineLazyServiceGetter(
   "aboutNewTabService",
   "@mozilla.org/browser/aboutnewtab-service;1",
   "nsIAboutNewTabService"
 );
 
 const IS_RELEASE_OR_BETA = AppConstants.RELEASE_OR_BETA;
 
 const DOWNLOADS_URL =
-  "chrome://browser/content/downloads/contentAreaDownloadsView.xul";
+  "chrome://browser/content/downloads/contentAreaDownloadsView.xhtml";
 const SEPARATE_PRIVILEGED_CONTENT_PROCESS_PREF =
   "browser.tabs.remote.separatePrivilegedContentProcess";
 const ACTIVITY_STREAM_DEBUG_PREF = "browser.newtabpage.activity-stream.debug";
 
 function cleanup() {
   Services.prefs.clearUserPref(SEPARATE_PRIVILEGED_CONTENT_PROCESS_PREF);
   Services.prefs.clearUserPref(ACTIVITY_STREAM_DEBUG_PREF);
   aboutNewTabService.resetNewTabURL();

--- a/browser/components/places/PlacesUIUtils.jsm
+++ b/browser/components/places/PlacesUIUtils.jsm
@@ -305,18 +305,18 @@ var PlacesUIUtils = {
   showBookmarkDialog(aInfo, aParentWindow) {
     // Preserve size attributes differently based on the fact the dialog has
     // a folder picker or not, since it needs more horizontal space than the
     // other controls.
     let hasFolderPicker =
       !("hiddenRows" in aInfo) || !aInfo.hiddenRows.includes("folderPicker");
     // Use a different chrome url to persist different sizes.
     let dialogURL = hasFolderPicker
-      ? "chrome://browser/content/places/bookmarkProperties2.xul"
-      : "chrome://browser/content/places/bookmarkProperties.xul";
+      ? "chrome://browser/content/places/bookmarkProperties2.xhtml"
+      : "chrome://browser/content/places/bookmarkProperties.xhtml";
 
     let features = "centerscreen,chrome,modal,resizable=yes";
 
     let topUndoEntry;
     let batchBlockingDeferred;
 
     // Set the transaction manager into batching mode.
     topUndoEntry = PlacesTransactions.topUndoEntry;

rename from browser/components/places/content/bookmarkProperties.xul
rename to browser/components/places/content/bookmarkProperties.xhtml

rename from browser/components/places/content/bookmarksSidebar.xul
rename to browser/components/places/content/bookmarksSidebar.xhtml

--- a/browser/components/places/content/historySidebar.js
+++ b/browser/components/places/content/historySidebar.js
@@ -119,17 +119,17 @@ function searchHistory(aInput) {
   options.resultType = resultType;
   options.includeHidden = !!aInput;
 
   if (gHistoryGrouping == "lastvisited") {
     TelemetryStopwatch.start("HISTORY_LASTVISITED_TREE_QUERY_TIME_MS");
   }
 
   // call load() on the tree manually
-  // instead of setting the place attribute in historySidebar.xul
+  // instead of setting the place attribute in historySidebar.xhtml
   // otherwise, we will end up calling load() twice
   gHistoryTree.load(query, options);
 
   if (gHistoryGrouping == "lastvisited") {
     TelemetryStopwatch.finish("HISTORY_LASTVISITED_TREE_QUERY_TIME_MS");
   }
 }
 

rename from browser/components/places/content/historySidebar.xul
rename to browser/components/places/content/historySidebar.xhtml

rename from browser/components/places/content/places.xul
rename to browser/components/places/content/places.xhtml

--- a/browser/components/places/jar.mn
+++ b/browser/components/places/jar.mn
@@ -1,23 +1,23 @@
 # This Source Code Form is subject to the terms of the Mozilla Public
 # License, v. 2.0. If a copy of the MPL was not distributed with this
 # file, You can obtain one at http://mozilla.org/MPL/2.0/.
 
 browser.jar:
 # Provide another URI for the bookmarkProperties dialog so we can persist the
 # attributes separately
-*   content/browser/places/bookmarkProperties2.xul       (content/bookmarkProperties.xul)
-*   content/browser/places/places.xul                    (content/places.xul)
+*   content/browser/places/bookmarkProperties2.xhtml     (content/bookmarkProperties.xhtml)
+*   content/browser/places/places.xhtml                  (content/places.xhtml)
     content/browser/places/places.js                     (content/places.js)
     content/browser/places/places.css                    (content/places.css)
-*   content/browser/places/bookmarkProperties.xul        (content/bookmarkProperties.xul)
+*   content/browser/places/bookmarkProperties.xhtml      (content/bookmarkProperties.xhtml)
     content/browser/places/bookmarkProperties.js         (content/bookmarkProperties.js)
     content/browser/places/places-menupopup.js           (content/places-menupopup.js)
     content/browser/places/places-tree.js                (content/places-tree.js)
     content/browser/places/controller.js                 (content/controller.js)
     content/browser/places/treeView.js                   (content/treeView.js)
     content/browser/places/browserPlacesViews.js         (content/browserPlacesViews.js)
-*   content/browser/places/historySidebar.xul            (content/historySidebar.xul)
+*   content/browser/places/historySidebar.xhtml          (content/historySidebar.xhtml)
     content/browser/places/historySidebar.js             (content/historySidebar.js)
-*   content/browser/places/bookmarksSidebar.xul          (content/bookmarksSidebar.xul)
+*   content/browser/places/bookmarksSidebar.xhtml        (content/bookmarksSidebar.xhtml)
     content/browser/places/bookmarksSidebar.js           (content/bookmarksSidebar.js)
     content/browser/places/editBookmark.js               (content/editBookmark.js)

--- a/browser/components/places/tests/browser/browser_bookmarksProperties.js
+++ b/browser/components/places/tests/browser/browser_bookmarksProperties.js
@@ -26,19 +26,19 @@ const ACTION_ADD = 1;
 
 // If action is ACTION_ADD, set type to one of those, to define what do you
 // want to create.
 const TYPE_FOLDER = 0;
 const TYPE_BOOKMARK = 1;
 
 const TEST_URL = "http://www.example.com/";
 
-const DIALOG_URL = "chrome://browser/content/places/bookmarkProperties.xul";
+const DIALOG_URL = "chrome://browser/content/places/bookmarkProperties.xhtml";
 const DIALOG_URL_MINIMAL_UI =
-  "chrome://browser/content/places/bookmarkProperties2.xul";
+  "chrome://browser/content/places/bookmarkProperties2.xhtml";
 
 const { BrowserWindowTracker } = ChromeUtils.import(
   "resource:///modules/BrowserWindowTracker.jsm"
 );
 var win = BrowserWindowTracker.getTopWindow();
 
 function add_bookmark(url) {
   return PlacesUtils.bookmarks.insert({

--- a/browser/components/places/tests/browser/head.js
+++ b/browser/components/places/tests/browser/head.js
@@ -6,17 +6,17 @@ ChromeUtils.defineModuleGetter(
 ChromeUtils.defineModuleGetter(
   this,
   "TestUtils",
   "resource://testing-common/TestUtils.jsm"
 );
 
 function openLibrary(callback, aLeftPaneRoot) {
   let library = window.openDialog(
-    "chrome://browser/content/places/places.xul",
+    "chrome://browser/content/places/places.xhtml",
     "",
     "chrome,toolbar=yes,dialog=no,resizable",
     aLeftPaneRoot
   );
   waitForFocus(function() {
     callback(library);
   }, library);
 

--- a/browser/components/preferences/in-content/privacy.js
+++ b/browser/components/preferences/in-content/privacy.js
@@ -1337,17 +1337,17 @@ var gPrivacyPane = {
     var ts = Preferences.get("privacy.sanitize.timeSpan");
     var timeSpanOrig = ts.value;
 
     if (aClearEverything) {
       ts.value = 0;
     }
 
     gSubDialog.open(
-      "chrome://browser/content/sanitize.xul",
+      "chrome://browser/content/sanitize.xhtml",
       "resizable=no",
       null,
       () => {
         // reset the timeSpan pref
         if (aClearEverything) {
           ts.value = timeSpanOrig;
         }
 

--- a/browser/components/privatebrowsing/test/browser/browser_privatebrowsing_nonbrowser.js
+++ b/browser/components/privatebrowsing/test/browser/browser_privatebrowsing_nonbrowser.js
@@ -3,17 +3,17 @@
 /**
  * Tests that we fire the last-pb-context-exited observer notification
  * when the last private browsing window closes, even if a chrome window
  * was opened from that private browsing window.
  */
 add_task(async function() {
   let win = await BrowserTestUtils.openNewBrowserWindow({ private: true });
   let chromeWin = win.open(
-    "chrome://browser/content/places/places.xul",
+    "chrome://browser/content/places/places.xhtml",
     "_blank",
     "chrome,extrachrome,menubar,resizable,scrollbars,status,toolbar"
   );
   await BrowserTestUtils.waitForEvent(chromeWin, "load");
   let obsPromise = TestUtils.topicObserved("last-pb-context-exited");
   await BrowserTestUtils.closeWindow(win);
   await obsPromise;
   Assert.ok(true, "Got the last-pb-context-exited notification");

rename from browser/components/shell/content/setDesktopBackground.xul
rename to browser/components/shell/content/setDesktopBackground.xhtml

--- a/browser/components/shell/jar.mn
+++ b/browser/components/shell/jar.mn
@@ -1,7 +1,7 @@
 # This Source Code Form is subject to the terms of the Mozilla Public
 # License, v. 2.0. If a copy of the MPL was not distributed with this
 # file, You can obtain one at http://mozilla.org/MPL/2.0/.
 
 browser.jar:
-*       content/browser/setDesktopBackground.xul              (content/setDesktopBackground.xul)
+*       content/browser/setDesktopBackground.xhtml            (content/setDesktopBackground.xhtml)
         content/browser/setDesktopBackground.js               (content/setDesktopBackground.js)

--- a/browser/components/urlbar/tests/browser/browser_dragdropURL.js
+++ b/browser/components/urlbar/tests/browser/browser_dragdropURL.js
@@ -63,17 +63,17 @@ add_task(async function checkDragURL() {
 });
 
 add_task(async function checkDragForbiddenURL() {
   await BrowserTestUtils.withNewTab(TEST_URL, function(browser) {
     // See also browser_removeUnsafeProtocolsFromURLBarPaste.js for other
     // examples. In general we trust that function, we pick some testcases to
     // ensure we disallow dropping trimmed text.
     for (let url of [
-      "chrome://browser/content/aboutDialog.xul",
+      "chrome://browser/content/aboutDialog.xhtml",
       "file:///",
       "javascript:",
       "javascript:void(0)",
       "java\r\ns\ncript:void(0)",
       " javascript:void(0)",
       "\u00A0java\nscript:void(0)",
       "javascript:document.domain",
       "javascript:javascript:alert('hi!')",

--- a/browser/components/urlbar/tests/ext/browser/browser_ext_urlbar_openClearHistoryDialog.js
+++ b/browser/components/urlbar/tests/ext/browser/browser_ext_urlbar_openClearHistoryDialog.js
@@ -39,17 +39,17 @@ async function checkExtension(isPrivate 
   let dialogPromise = new Promise((resolve, reject) => {
     function onOpen(subj, topic, data) {
       if (topic == "domwindowopened" && subj instanceof Ci.nsIDOMWindow) {
         subj.addEventListener(
           "load",
           function() {
             if (
               subj.document.documentURI ==
-              "chrome://browser/content/sanitize.xul"
+              "chrome://browser/content/sanitize.xhtml"
             ) {
               Services.ww.unregisterNotification(onOpen);
               Assert.ok(true, "Observed Clear Recent History window open");
               // On macOS the Clear Recent History dialog gets opened as an
               // app-modal window, so its opener is null.
               is(
                 subj.opener,
                 AppConstants.platform == "macosx" ? null : window,

--- a/browser/installer/allowed-dupes.mn
+++ b/browser/installer/allowed-dupes.mn
@@ -12,18 +12,18 @@ updater.app/Contents/PkgInfo
 browser/chrome.manifest
 # browser branding / themes is bug 1313106
 browser/chrome/browser/content/branding/icon16.png
 browser/chrome/browser/content/branding/icon32.png
 browser/chrome/browser/content/branding/icon48.png
 browser/chrome/browser/content/branding/icon64.png
 browser/chrome/browser/content/browser/defaultthemes/5.header.png
 browser/chrome/browser/content/browser/extension.svg
-browser/chrome/browser/content/browser/places/bookmarkProperties.xul
-browser/chrome/browser/content/browser/places/bookmarkProperties2.xul
+browser/chrome/browser/content/browser/places/bookmarkProperties.xhtml
+browser/chrome/browser/content/browser/places/bookmarkProperties2.xhtml
 browser/chrome/browser/skin/classic/browser/addons/addon-install-confirm.svg
 browser/chrome/browser/skin/classic/browser/connection-secure.svg
 browser/chrome/browser/skin/classic/browser/controlcenter/warning-gray.svg
 # devtools reduction is bug 1311178
 browser/chrome/devtools/content/dom/content/dom-view.css
 browser/chrome/devtools/content/dom/index.html
 browser/chrome/devtools/content/dom/main.js
 browser/chrome/devtools/content/framework/toolbox-options.js

--- a/browser/modules/Sanitizer.jsm
+++ b/browser/modules/Sanitizer.jsm
@@ -118,17 +118,17 @@ var Sanitizer = {
    */
   showUI(parentWindow) {
     let win =
       AppConstants.platform == "macosx"
         ? null // make this an app-modal window on Mac
         : parentWindow;
     Services.ww.openWindow(
       win,
-      "chrome://browser/content/sanitize.xul",
+      "chrome://browser/content/sanitize.xhtml",
       "Sanitize",
       "chrome,titlebar,dialog,centerscreen,modal",
       null
     );
   },
 
   /**
    * Performs startup tasks:

--- a/browser/modules/webrtcUI.jsm
+++ b/browser/modules/webrtcUI.jsm
@@ -1327,17 +1327,18 @@ function removePrompt(aBrowser, aCallId)
   );
   if (notification && notification.callID == aCallId) {
     notification.remove();
   }
 }
 
 function getGlobalIndicator() {
   if (AppConstants.platform != "macosx") {
-    const INDICATOR_CHROME_URI = "chrome://browser/content/webrtcIndicator.xul";
+    const INDICATOR_CHROME_URI =
+      "chrome://browser/content/webrtcIndicator.xhtml";
     const features = "chrome,dialog=yes,titlebar=no,popup=yes";
 
     return Services.ww.openWindow(
       null,
       INDICATOR_CHROME_URI,
       "_blank",
       features,
       []

--- a/dom/base/nsFrameLoader.cpp
+++ b/dom/base/nsFrameLoader.cpp
@@ -2547,17 +2547,17 @@ bool nsFrameLoader::TryRemoteBrowserInte
       if (NS_FAILED(parentURI->GetSpecIgnoringRef(specIgnoringRef))) {
         return false;
       }
 
       if (!(specIgnoringRef.EqualsLiteral("about:addons") ||
             specIgnoringRef.EqualsLiteral(
                 "chrome://mozapps/content/extensions/aboutaddons.html") ||
             specIgnoringRef.EqualsLiteral(
-                "chrome://browser/content/webext-panels.xul"))) {
+                "chrome://browser/content/webext-panels.xhtml"))) {
         return false;
       }
     }
 
     if (!mOwnerContent->IsXULElement()) {
       return false;
     }
 

--- a/testing/firefox-ui/tests/puppeteer/test_windows.py
+++ b/testing/firefox-ui/tests/puppeteer/test_windows.py
@@ -63,17 +63,17 @@ class TestWindows(PuppeteerMixin, Marion
         self.browser.switch_to()
 
         self.assertEqual(len(self.puppeteer.windows.all), 1)
 
     def test_switch_to_unknown_window_type(self):
         def open_by_js(_):
             with self.marionette.using_context('chrome'):
                 self.marionette.execute_script("""
-                  window.open('chrome://browser/content/safeMode.xul', '_blank',
+                  window.open('chrome://browser/content/safeMode.xhtml', '_blank',
                               'chrome,centerscreen,resizable=no');
                 """)
 
         win = self.browser.open_window(callback=open_by_js, expected_window_class=BaseWindow)
         win.close()
         self.browser.switch_to()
 
 

--- a/toolkit/mozapps/update/tests/browser/head.js
+++ b/toolkit/mozapps/update/tests/browser/head.js
@@ -505,17 +505,17 @@ function waitForAboutDialog() {
   return new Promise(resolve => {
     var listener = {
       onOpenWindow: aXULWindow => {
         debugDump("About dialog shown...");
         Services.wm.removeListener(listener);
 
         async function aboutDialogOnLoad() {
           domwindow.removeEventListener("load", aboutDialogOnLoad, true);
-          let chromeURI = "chrome://browser/content/aboutDialog.xul";
+          let chromeURI = "chrome://browser/content/aboutDialog.xhtml";
           is(
             domwindow.document.location.href,
             chromeURI,
             "About dialog appeared"
           );
           resolve(domwindow);
         }