--- a/browser/base/content/browser-places.js
+++ b/browser/base/content/browser-places.js
@@ -123,23 +123,16 @@ var StarUI = {
                 aEvent.target.classList.contains("expander-down") ||
                 aEvent.target.id == "editBMPanel_newFolderButton")  {
               //XXX Why is this necessary? The defaultPrevented check should
               //    be enough.
               break;
             }
             this.panel.hidePopup();
             break;
-          // This case is for catching character-generating keypresses
-          case 0:
-            let accessKey = document.getElementById("key_close");
-            if (eventMatchesKey(aEvent, accessKey)) {
-                this.panel.hidePopup();
-            }
-            break;
         }
         break;
       case "mouseout":
         // Explicit fall-through
       case "popupshown":
         // Don't handle events for descendent elements.
         if (aEvent.target != aEvent.currentTarget) {
           break;

--- a/browser/base/content/browser.js
+++ b/browser/base/content/browser.js
@@ -1351,25 +1351,16 @@ var gBrowserInit = {
                         slot.status != Ci.nsIPKCS11Slot.SLOT_READY;
         if (mpEnabled) {
           Services.telemetry.getHistogramById("MASTER_PASSWORD_ENABLED").add(mpEnabled);
         }
       }, 5000);
 
       PanicButtonNotifier.init();
     });
-
-    gBrowser.tabContainer.addEventListener("TabSelect", function() {
-      for (let panel of document.querySelectorAll("panel[tabspecific='true']")) {
-        if (panel.state == "open") {
-          panel.hidePopup();
-        }
-      }
-    });
-
     this.delayedStartupFinished = true;
 
     Services.obs.notifyObservers(window, "browser-delayed-startup-finished", "");
     TelemetryTimestamps.add("delayedStartupFinished");
   },
 
   // Returns the URI(s) to load at startup.
   _getUriToLoad: function () {

--- a/browser/base/content/browser.xul
+++ b/browser/base/content/browser.xul
@@ -172,17 +172,16 @@
       <description/>
     </panel>
 
     <panel id="editBookmarkPanel"
            type="arrow"
            orient="vertical"
            ignorekeys="true"
            hidden="true"
-           tabspecific="true"
            onpopupshown="StarUI.panelShown(event);"
            aria-labelledby="editBookmarkPanelTitle">
       <row id="editBookmarkPanelHeader" align="center" hidden="true">
         <vbox align="center">
           <image id="editBookmarkPanelStarIcon"/>
         </vbox>
         <vbox>
           <label id="editBookmarkPanelTitle"/>

--- a/browser/base/content/test/general/browser_utilityOverlay.js
+++ b/browser/base/content/test/general/browser_utilityOverlay.js
@@ -1,14 +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/. */
 
 const gTests = [
-  test_eventMatchesKey,
   test_getTopWin,
   test_getBoolPref,
   test_openNewTabWith,
   test_openUILink
 ];
 
 function test () {
   waitForExplicitFinish();
@@ -21,59 +20,16 @@ function runNextTest() {
     info("Running " + testFun.name);
     testFun()
   }
   else {
     finish();
   }
 }
 
-function test_eventMatchesKey() {
-  let eventMatchResult;
-  document.addEventListener("keypress", function(e) {
-      e.stopPropagation();
-      e.preventDefault();
-      eventMatchResult = eventMatchesKey(e, key);
-  });
-
-  let key = document.createElement("key");
-  let keyset = document.getElementById("mainKeyset");
-  key.setAttribute("key", "t");
-  key.setAttribute("modifiers", "accel");
-  keyset.appendChild(key);
-  EventUtils.synthesizeKey("t", {accelKey: true});
-  is(eventMatchResult, true, "eventMatchesKey: one modifier");
-  keyset.removeChild(key);
-
-  key = document.createElement("key");
-  key.setAttribute("key", "g");
-  key.setAttribute("modifiers", "accel,shift");
-  keyset.appendChild(key);
-  EventUtils.synthesizeKey("g", {accelKey: true, shiftKey: true});
-  is(eventMatchResult, true, "eventMatchesKey: combination modifiers");
-  keyset.removeChild(key);
-
-  key = document.createElement("key");
-  key.setAttribute("key", "w");
-  key.setAttribute("modifiers", "accel");
-  keyset.appendChild(key);
-  EventUtils.synthesizeKey("f", {accelKey: true});
-  is(eventMatchResult, false, "eventMatchesKey: mismatch keys");
-  keyset.removeChild(key);
-
-  key = document.createElement("key");
-  key.setAttribute("keycode", "VK_DELETE");
-  keyset.appendChild(key);
-  EventUtils.synthesizeKey("VK_DELETE", {accelKey: true});
-  is(eventMatchResult, false, "eventMatchesKey: mismatch modifiers");
-  keyset.removeChild(key);
-
-  runNextTest();
-}
-
 function test_getTopWin() {
   is(getTopWin(), window, "got top window");
   runNextTest();
 }
 
 
 function test_getBoolPref() {
   is(getBoolPref("browser.search.openintab", false), false, "getBoolPref");

--- a/browser/base/content/test/tabPrompts/browser.ini
+++ b/browser/base/content/test/tabPrompts/browser.ini
@@ -1,4 +1,3 @@
-[browser_closeTabSpecificPanels.js]
 [browser_multiplePrompts.js]
 [browser_openPromptInBackgroundTab.js]
 support-files = openPromptOffTimeout.html

deleted file mode 100644
--- a/browser/base/content/test/tabPrompts/browser_closeTabSpecificPanels.js
+++ /dev/null
@@ -1,41 +0,0 @@
-"use strict";
-
-/*
- * This test creates multiple panels, one that has been tagged as specific to its tab's content
- * and one that isn't. When a tab loses focus, panel specific to that tab should close.
- * The non-specific panel should remain open.
- *
- */
-
-add_task(function*() {
-  let tab1 = gBrowser.addTab("http://mochi.test:8888/#0");
-  let tab2 = gBrowser.addTab("http://mochi.test:8888/#1");
-  let specificPanel = document.createElement("panel");
-  specificPanel.setAttribute("tabspecific", "true");
-  let generalPanel = document.createElement("panel");
-  let anchor = document.getElementById(CustomizableUI.AREA_NAVBAR);
-
-  anchor.appendChild(specificPanel);
-  anchor.appendChild(generalPanel);
-  is(specificPanel.state, "closed", "specificPanel starts as closed");
-  is(generalPanel.state, "closed", "generalPanel starts as closed");
-
-  let specificPanelPromise = BrowserTestUtils.waitForEvent(specificPanel, "popupshown");
-  specificPanel.openPopupAtScreen(210, 210);
-  yield specificPanelPromise;
-  is(specificPanel.state, "open", "specificPanel has been opened");
-
-  let generalPanelPromise = BrowserTestUtils.waitForEvent(generalPanel, "popupshown");
-  generalPanel.openPopupAtScreen(510,510);
-  yield generalPanelPromise;
-  is(generalPanel.state, "open", "generalPanel has been opened");
-
-  gBrowser.tabContainer.advanceSelectedTab(-1, true);
-  is(specificPanel.state, "closed", "specificPanel panel is closed after its tab loses focus");
-  is(generalPanel.state, "open", "generalPanel is still open after tab switch");
-
-  specificPanel.remove();
-  generalPanel.remove();
-  gBrowser.removeTab(tab1);
-  gBrowser.removeTab(tab2);
-});

--- a/browser/base/content/utilityOverlay.js
+++ b/browser/base/content/utilityOverlay.js
@@ -473,56 +473,16 @@ function closeMenus(node)
     if (node.namespaceURI == "http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul"
     && (node.tagName == "menupopup" || node.tagName == "popup"))
       node.hidePopup();
 
     closeMenus(node.parentNode);
   }
 }
 
-/** This function takes in a key element and compares it to the keys pressed during an event.
- *
- * @param aEvent
- *        The KeyboardEvent event you want to compare against your key.
- *
- * @param aKey
- *        The <key> element checked to see if it was called in aEvent.
- *        For example, aKey can be a variable set to document.getElementById("key_close")
- *        to check if the close command key was pressed in aEvent.
-*/
-function eventMatchesKey(aEvent, aKey)
-{
-  let keyPressed = aKey.getAttribute("key").toLowerCase();
-  let keyModifiers = aKey.getAttribute("modifiers");
-  let modifiers = ["Alt", "Control", "Meta", "Shift"];
-
-  if (aEvent.key != keyPressed) {
-    return false;
-  }
-  let eventModifiers = modifiers.filter(modifier => aEvent.getModifierState(modifier));
-  // Check if aEvent has a modifier and aKey doesn't
-  if (eventModifiers.length > 0 && keyModifiers.length == 0) {
-     return false;
-  }
-  // Check whether aKey's modifiers match aEvent's modifiers
-  if (keyModifiers) {
-    keyModifiers = keyModifiers.split(/[\s,]+/);
-    // Capitalize first letter of aKey's modifers to compare to aEvent's modifier
-    keyModifiers.forEach(function(modifier, index) {
-      if (modifier == "accel") {
-        keyModifiers[index] = AppConstants.platform == "macosx" ?  "Meta" : "Control";
-      } else {
-        keyModifiers[index] = modifier[0].toUpperCase() + modifier.slice(1);
-      }
-    });
-    return modifiers.every(modifier => keyModifiers.includes(modifier) == aEvent.getModifierState(modifier));
-  }
-  return true;
-}
-
 // Gather all descendent text under given document node.
 function gatherTextUnder ( root )
 {
   var text = "";
   var node = root.firstChild;
   var depth = 1;
   while ( node && depth > 0 ) {
     // See if this node is text.

--- a/browser/components/customizableui/CustomizableUI.jsm
+++ b/browser/components/customizableui/CustomizableUI.jsm
@@ -1353,19 +1353,16 @@ var CustomizableUIInternal = {
       node.setAttribute("id", aWidget.id);
       node.setAttribute("widget-id", aWidget.id);
       node.setAttribute("widget-type", aWidget.type);
       if (aWidget.disabled) {
         node.setAttribute("disabled", true);
       }
       node.setAttribute("removable", aWidget.removable);
       node.setAttribute("overflows", aWidget.overflows);
-      if (aWidget.tabSpecific) {
-        node.setAttribute("tabspecific", aWidget.tabSpecific);
-      }
       node.setAttribute("label", this.getLocalizedProperty(aWidget, "label"));
       let additionalTooltipArguments = [];
       if (aWidget.shortcutId) {
         let keyEl = aDocument.getElementById(aWidget.shortcutId);
         if (keyEl) {
           additionalTooltipArguments.push(ShortcutUtils.prettifyShortcut(keyEl));
         } else {
           log.error("Key element with id '" + aWidget.shortcutId + "' for widget '" + aWidget.id +
@@ -2308,17 +2305,16 @@ var CustomizableUIInternal = {
       implementation: aData,
       source: aSource || CustomizableUI.SOURCE_EXTERNAL,
       instances: new Map(),
       currentArea: null,
       removable: true,
       overflows: true,
       defaultArea: null,
       shortcutId: null,
-      tabSpecific: false,
       tooltiptext: null,
       showInPrivateBrowsing: true,
       _introducedInVersion: -1,
     };
 
     if (typeof aData.id != "string" || !/^[a-z0-9-_]{1,}$/i.test(aData.id)) {
       log.error("Given an illegal id in normalizeWidget: " + aData.id);
       return null;
@@ -2339,17 +2335,17 @@ var CustomizableUIInternal = {
 
     const kOptStringProps = ["label", "tooltiptext", "shortcutId"];
     for (let prop of kOptStringProps) {
       if (typeof aData[prop] == "string") {
         widget[prop] = aData[prop];
       }
     }
 
-    const kOptBoolProps = ["removable", "showInPrivateBrowsing", "overflows", "tabSpecific"];
+    const kOptBoolProps = ["removable", "showInPrivateBrowsing", "overflows"];
     for (let prop of kOptBoolProps) {
       if (typeof aData[prop] == "boolean") {
         widget[prop] = aData[prop];
       }
     }
 
     // When we normalize builtin widgets, areas have not yet been registered:
     if (aData.defaultArea &&

--- a/browser/components/customizableui/content/panelUI.js
+++ b/browser/components/customizableui/content/panelUI.js
@@ -330,19 +330,16 @@ const PanelUI = {
         return;
       }
 
       let tempPanel = document.createElement("panel");
       tempPanel.setAttribute("type", "arrow");
       tempPanel.setAttribute("id", "customizationui-widget-panel");
       tempPanel.setAttribute("class", "cui-widget-panel");
       tempPanel.setAttribute("viewId", aViewId);
-      if (aAnchor.getAttribute("tabspecific")) {
-        tempPanel.setAttribute("tabspecific", true);
-      }
       if (this._disableAnimations) {
         tempPanel.setAttribute("animate", "false");
       }
       tempPanel.setAttribute("context", "");
       document.getElementById(CustomizableUI.AREA_NAVBAR).appendChild(tempPanel);
       // If the view has a footer, set a convenience class on the panel.
       tempPanel.classList.toggle("cui-widget-panelWithFooter",
                                  viewNode.querySelector(".panel-subview-footer"));

--- a/browser/extensions/pocket/bootstrap.js
+++ b/browser/extensions/pocket/bootstrap.js
@@ -135,17 +135,16 @@ function CreatePocketWidget(reason) {
   // if upgrading from builtin version and the button was placed in ui,
   // seenWidget will not be null
   let seenWidget = CustomizableUI.getPlacementOfWidget("pocket-button", false, true);
   let pocketButton = {
     id: "pocket-button",
     defaultArea: CustomizableUI.AREA_NAVBAR,
     introducedInVersion: "pref",
     type: "view",
-    tabSpecific: true,
     viewId: "PanelUI-pocketView",
     label: gPocketBundle.GetStringFromName("pocket-button.label"),
     tooltiptext: gPocketBundle.GetStringFromName("pocket-button.tooltiptext"),
     // Use forwarding functions here to avoid loading Pocket.jsm on startup:
     onViewShowing: function() {
       return Pocket.onPanelViewShowing.apply(this, arguments);
     },
     onViewHiding: function() {