new file mode 100644
--- /dev/null
+++ b/toolkit/mozapps/downloads/tests/browser/.eslintrc.js
@@ -0,0 +1,9 @@
+"use strict";
+
+module.exports = {
+  "extends": [
+    "plugin:mozilla/browser-test",
+    "plugin:mozilla/mochitest-test",
+  ]
+};
+

new file mode 100644
--- /dev/null
+++ b/toolkit/mozapps/downloads/tests/browser/browser.ini
@@ -0,0 +1,9 @@
+[DEFAULT]
+support-files =
+  unknownContentType_dialog_layout_data.pif
+  unknownContentType_dialog_layout_data.pif^headers^
+  unknownContentType_dialog_layout_data.txt
+  unknownContentType_dialog_layout_data.txt^headers^
+
+[browser_unknownContentType_delayedbutton.js]
+[browser_unknownContentType_dialog_layout.js]
\ No newline at end of file

rename from toolkit/mozapps/downloads/tests/chrome/test_unknownContentType_delayedbutton.xul
rename to toolkit/mozapps/downloads/tests/browser/browser_unknownContentType_delayedbutton.js
--- a/toolkit/mozapps/downloads/tests/chrome/test_unknownContentType_delayedbutton.xul
+++ b/toolkit/mozapps/downloads/tests/browser/browser_unknownContentType_delayedbutton.js
@@ -1,114 +1,83 @@
-<?xml version="1.0"?>
-<!-- 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/.  -->
-<!--
- * The unknownContentType popup can have two different layouts depending on
- * whether a helper application can be selected or not.
- * This tests that both layouts have correct collapsed elements.
--->
+/* 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/. */
+
+ChromeUtils.import("resource://gre/modules/PromiseUtils.jsm", this);
+
+const UCT_URI = "chrome://mozapps/content/downloads/unknownContentType.xul";
+const LOAD_URI = "http://mochi.test:8888/browser/toolkit/mozapps/downloads/tests/browser/unknownContentType_dialog_layout_data.txt";
 
-<window title="Unknown Content Type Dialog Test"
-        xmlns="http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul"
-        onload="doTest()">
-
-  <script type="application/javascript"
-          src="chrome://mochikit/content/tests/SimpleTest/SimpleTest.js"/>
+const DIALOG_DELAY = Services.prefs.getIntPref("security.dialog_enable_delay") + 200;
 
-  <script type="application/javascript"><![CDATA[
-    ChromeUtils.import("resource://gre/modules/Services.jsm");
-    ChromeUtils.import("resource://gre/modules/Promise.jsm");
+let UCTObserver = {
+  opened: PromiseUtils.defer(),
+  closed: PromiseUtils.defer(),
 
-    const UCT_URI = "chrome://mozapps/content/downloads/unknownContentType.xul";
-    const LOAD_URI = "http://mochi.test:8888/chrome/toolkit/mozapps/downloads/tests/chrome/unknownContentType_dialog_layout_data.txt";
-
-    const DIALOG_DELAY = Services.prefs.getIntPref("security.dialog_enable_delay") + 200;
+  observe(aSubject, aTopic, aData) {
+    let win = aSubject.QueryInterface(Ci.nsIDOMEventTarget);
 
-    let UCTObserver = {
-      opened: Promise.defer(),
-      closed: Promise.defer(),
-
-      observe: function(aSubject, aTopic, aData) {
-        let win = aSubject.QueryInterface(Ci.nsIDOMEventTarget);
-
-        switch (aTopic) {
-          case "domwindowopened":
-            win.addEventListener("load", function onLoad(event) {
-              win.removeEventListener("load", onLoad, false);
+    switch (aTopic) {
+      case "domwindowopened":
+        win.addEventListener("load", function onLoad(event) {
+          // Let the dialog initialize
+          SimpleTest.executeSoon(function() {
+          UCTObserver.opened.resolve(win);
+          });
+        }, {once: true});
+        break;
 
-              // Let the dialog initialize
-              SimpleTest.executeSoon(function() {
-                UCTObserver.opened.resolve(win);
-              });
-            }, false);
-            break;
-
-          case "domwindowclosed":
-            if (win.location == UCT_URI) {
-              this.closed.resolve();
-            }
-            break;
+      case "domwindowclosed":
+        if (win.location == UCT_URI) {
+          this.closed.resolve();
         }
-      }
-    };
+        break;
+    }
+  }
+};
 
-    Services.ww.registerNotification(UCTObserver);
-    SimpleTest.waitForExplicitFinish();
-    SimpleTest.requestFlakyTimeout("This test is testing a timing-based feature, so it really needs to wait a certain amount of time to verify that the feature worked.");
+function waitDelay(delay) {
+  return new Promise((resolve, reject) => {
+    /* eslint-disable mozilla/no-arbitrary-setTimeout */
+    window.setTimeout(resolve, delay);
+  });
+}
 
-    function waitDelay(delay) {
-      return new Promise((resolve, reject) => {
-        window.setTimeout(resolve, delay);
-      });
-    }
-
-    function doTest() {
-      (async function test_aboutCrashed() {
-        let frame = document.getElementById("testframe");
-        frame.setAttribute("src", LOAD_URI);
+add_task(async function test_unknownContentType_delayedbutton() {
+  Services.ww.registerNotification(UCTObserver);
 
-        let uctWindow = await UCTObserver.opened.promise;
-        let ok = uctWindow.document.documentElement.getButton("accept");
-
-        SimpleTest.is(ok.disabled, true, "button started disabled");
-
-        await waitDelay(DIALOG_DELAY);
+  await BrowserTestUtils.withNewTab({
+    gBrowser,
+    url: LOAD_URI,
+  }, async function() {
+    let uctWindow = await UCTObserver.opened.promise;
+    let ok = uctWindow.document.documentElement.getButton("accept");
 
-        SimpleTest.is(ok.disabled, false, "button was enabled");
+    SimpleTest.is(ok.disabled, true, "button started disabled");
 
-        focusOutOfDialog = SimpleTest.promiseFocus(window);
-        window.focus();
-        await focusOutOfDialog;
+    await waitDelay(DIALOG_DELAY);
 
-        SimpleTest.is(ok.disabled, true, "button was disabled");
+    SimpleTest.is(ok.disabled, false, "button was enabled");
 
-        focusOnDialog = SimpleTest.promiseFocus(uctWindow);
-        uctWindow.focus();
-        await focusOnDialog;
+    let focusOutOfDialog = SimpleTest.promiseFocus(window);
+    window.focus();
+    await focusOutOfDialog;
 
-        SimpleTest.is(ok.disabled, true, "button remained disabled");
+    SimpleTest.is(ok.disabled, true, "button was disabled");
 
-        await waitDelay(DIALOG_DELAY);
-        SimpleTest.is(ok.disabled, false, "button re-enabled after delay");
+    let focusOnDialog = SimpleTest.promiseFocus(uctWindow);
+    uctWindow.focus();
+    await focusOnDialog;
 
-        uctWindow.document.documentElement.cancelDialog();
-        await UCTObserver.closed.promise;
+    SimpleTest.is(ok.disabled, true, "button remained disabled");
 
-        Services.ww.unregisterNotification(UCTObserver);
-        uctWindow = null;
-        UCTObserver = null;
-        SimpleTest.finish();
-      })();
-    }
-  ]]></script>
+    await waitDelay(DIALOG_DELAY);
+    SimpleTest.is(ok.disabled, false, "button re-enabled after delay");
+
+    uctWindow.document.documentElement.cancelDialog();
+    await UCTObserver.closed.promise;
 
-  <body xmlns="http://www.w3.org/1999/xhtml">
-    <p id="display"></p>
-    <div id="content" style="display:none;"></div>
-    <pre id="test"></pre>
-  </body>
-
-  <iframe xmlns="http://www.w3.org/1999/xhtml"
-          id="testframe">
-  </iframe>
-</window>
+    Services.ww.unregisterNotification(UCTObserver);
+    uctWindow = null;
+    UCTObserver = null;
+  });
+});

rename from toolkit/mozapps/downloads/tests/chrome/test_unknownContentType_dialog_layout.xul
rename to toolkit/mozapps/downloads/tests/browser/browser_unknownContentType_dialog_layout.js
--- a/toolkit/mozapps/downloads/tests/chrome/test_unknownContentType_dialog_layout.xul
+++ b/toolkit/mozapps/downloads/tests/browser/browser_unknownContentType_dialog_layout.js
@@ -1,106 +1,82 @@
-<?xml version="1.0"?>
-<!-- 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/.  -->
-<!--
+/* 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/. */
+
+/*
  * The unknownContentType popup can have two different layouts depending on
  * whether a helper application can be selected or not.
  * This tests that both layouts have correct collapsed elements.
--->
-
-<window title="Unknown Content Type Dialog Test"
-        xmlns="http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul"
-        onload="init()">
+ */
 
-  <script type="application/javascript"
-          src="chrome://mochikit/content/tests/SimpleTest/SimpleTest.js"/>
-
-  <script type="application/javascript">
-  <![CDATA[
 const UCT_URI = "chrome://mozapps/content/downloads/unknownContentType.xul";
 
-let testIndex = -1;
 let tests = [
   { // This URL will trigger the simple UI, where only the Save an Cancel buttons are available
-    url: "http://mochi.test:8888/chrome/toolkit/mozapps/downloads/tests/chrome/unknownContentType_dialog_layout_data.pif",
+    url: "http://mochi.test:8888/browser/toolkit/mozapps/downloads/tests/browser/unknownContentType_dialog_layout_data.pif",
     elements: {
       basicBox: { collapsed: false },
       normalBox: { collapsed: true }
     }
   },
   { // This URL will trigger the full UI
-    url: "http://mochi.test:8888/chrome/toolkit/mozapps/downloads/tests/chrome/unknownContentType_dialog_layout_data.txt",
+    url: "http://mochi.test:8888/browser/toolkit/mozapps/downloads/tests/browser/unknownContentType_dialog_layout_data.txt",
     elements: {
       basicBox: { collapsed: true },
       normalBox: { collapsed: false }
     }
   }
 ];
 
-let ww = Cc["@mozilla.org/embedcomp/window-watcher;1"].
-         getService(Ci.nsIWindowWatcher);
-
-SimpleTest.waitForExplicitFinish();
+add_task(async function test_unknownContentType_dialog_layout() {
+  for (let test of tests) {
+    let UCTObserver = {
+      opened: PromiseUtils.defer(),
+      closed: PromiseUtils.defer(),
 
-let windowObserver = {
-  observe: function(aSubject, aTopic, aData) {
-    let win = aSubject.QueryInterface(Ci.nsIDOMEventTarget);
-
-    if (aTopic == "domwindowclosed") {
-      if (win.location == UCT_URI)
-        loadNextTest();
-      return;
-    }
+      observe(aSubject, aTopic, aData) {
+        let win = aSubject.QueryInterface(Ci.nsIDOMEventTarget);
 
-    // domwindowopened
-    win.addEventListener("load", function onLoad(event) {
-      win.removeEventListener("load", onLoad, false);
-
-      // Let the dialog initialize
-      SimpleTest.executeSoon(function() {
-        checkWindow(win);
-      });
-    }, false);
-  }
-};
-
-function init() {
-  ww.registerNotification(windowObserver);
-  loadNextTest();
-}
+        switch (aTopic) {
+          case "domwindowopened":
+            win.addEventListener("load", function onLoad(event) {
+              // Let the dialog initialize
+              SimpleTest.executeSoon(function() {
+                UCTObserver.opened.resolve(win);
+              });
+            }, {once: true});
+            break;
 
-function loadNextTest() {
-  if (!tests[++testIndex]) {
-    ww.unregisterNotification(windowObserver);
-    SimpleTest.finish();
-    return;
-  }
-  let frame = document.getElementById("testframe");
-  frame.setAttribute("src", tests[testIndex].url);
-}
+          case "domwindowclosed":
+            if (win.location == UCT_URI) {
+              this.closed.resolve();
+            }
+            break;
+        }
+      }
+    };
+
+    Services.ww.registerNotification(UCTObserver);
+    await BrowserTestUtils.withNewTab({
+      gBrowser,
+      url: test.url,
+    }, async function() {
+      let uctWindow = await UCTObserver.opened.promise;
 
-function checkWindow(win) {
-  for (let [id, props] of Object.entries(tests[testIndex].elements)) {
-    let elem = win.dialog.dialogElement(id);
-    for (let [prop, value] of Object.entries(props)) {
-      is(elem[prop], value,
-         "Element with id " + id + " has property " +
-         prop + " set to " + value);
-    }
-  }
-  win.document.documentElement.cancelDialog();
-}
+      for (let [id, props] of Object.entries(test.elements)) {
+        let elem = uctWindow.dialog.dialogElement(id);
+        for (let [prop, value] of Object.entries(props)) {
+          SimpleTest.is(elem[prop], value,
+             "Element with id " + id + " has property " +
+             prop + " set to " + value);
+        }
+      }
+      let focusOnDialog = SimpleTest.promiseFocus(uctWindow);
+      uctWindow.focus();
+      await focusOnDialog;
 
-  ]]>
-  </script>
-
-  <body xmlns="http://www.w3.org/1999/xhtml">
-    <p id="display"></p>
-    <div id="content" style="display:none;"></div>
-    <pre id="test"></pre>
-  </body>
-
-  <iframe xmlns="http://www.w3.org/1999/xhtml"
-          id="testframe">
-  </iframe>
-</window>
+      uctWindow.document.documentElement.cancelDialog();
+      uctWindow = null;
+      Services.ww.unregisterNotification(UCTObserver);
+    });
+  }
+});

rename from toolkit/mozapps/downloads/tests/chrome/unknownContentType_dialog_layout_data.pif
rename to toolkit/mozapps/downloads/tests/browser/unknownContentType_dialog_layout_data.pif

rename from toolkit/mozapps/downloads/tests/chrome/unknownContentType_dialog_layout_data.pif^headers^
rename to toolkit/mozapps/downloads/tests/browser/unknownContentType_dialog_layout_data.pif^headers^

rename from toolkit/mozapps/downloads/tests/chrome/unknownContentType_dialog_layout_data.txt
rename to toolkit/mozapps/downloads/tests/browser/unknownContentType_dialog_layout_data.txt

rename from toolkit/mozapps/downloads/tests/chrome/unknownContentType_dialog_layout_data.txt^headers^
rename to toolkit/mozapps/downloads/tests/browser/unknownContentType_dialog_layout_data.txt^headers^

deleted file mode 100644
--- a/toolkit/mozapps/downloads/tests/chrome/.eslintrc.js
+++ /dev/null
@@ -1,7 +0,0 @@
-"use strict";
-
-module.exports = {
-  "extends": [
-    "plugin:mozilla/chrome-test"
-  ]
-};

deleted file mode 100644
--- a/toolkit/mozapps/downloads/tests/chrome/chrome.ini
+++ /dev/null
@@ -1,10 +0,0 @@
-[DEFAULT]
-skip-if = os == 'android'
-support-files =
-  unknownContentType_dialog_layout_data.pif
-  unknownContentType_dialog_layout_data.pif^headers^
-  unknownContentType_dialog_layout_data.txt
-  unknownContentType_dialog_layout_data.txt^headers^
-
-[test_unknownContentType_delayedbutton.xul]
-[test_unknownContentType_dialog_layout.xul]

--- a/toolkit/mozapps/downloads/tests/moz.build
+++ b/toolkit/mozapps/downloads/tests/moz.build
@@ -1,8 +1,8 @@
 # -*- Mode: python; indent-tabs-mode: nil; tab-width: 40 -*-
 # vim: set filetype=python:
 # 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/.
 
 XPCSHELL_TESTS_MANIFESTS += ['unit/xpcshell.ini']
-MOCHITEST_CHROME_MANIFESTS += ['chrome/chrome.ini']
+BROWSER_CHROME_MANIFESTS += ['browser/browser.ini']