--- a/browser/base/content/browser.js
+++ b/browser/base/content/browser.js
@@ -22,17 +22,18 @@ Cu.import("resource://gre/modules/Notifi
           GMPInstallManager:false, LightweightThemeManager:false, Log:false,
           LoginManagerParent:false, NewTabUtils:false, PageThumbs:false,
           PluralForm:false, Preferences:false, PrivateBrowsingUtils:false,
           ProcessHangMonitor:false, PromiseUtils:false, ReaderMode:false,
           ReaderParent:false, RecentWindow:false, SessionStore:false,
           ShortcutUtils:false, SimpleServiceDiscovery:false, SitePermissions:false,
           Social:false, TabCrashHandler:false, Task:false, TelemetryStopwatch:false,
           Translation:false, UITour:false, UpdateUtils:false, Weave:false,
-          fxAccounts:false, gDevTools:false, gDevToolsBrowser:false, webrtcUI:false
+          fxAccounts:false, gDevTools:false, gDevToolsBrowser:false, webrtcUI:false,
+          URLBarZoom:false
  */
 
 /**
  * IF YOU ADD OR REMOVE FROM THIS LIST, PLEASE UPDATE THE LIST ABOVE AS WELL.
  * XXX Bug 1325373 is for making eslint detect these automatically.
  */
 [
   ["AboutHome", "resource:///modules/AboutHome.jsm"],
@@ -69,21 +70,22 @@ Cu.import("resource://gre/modules/Notifi
   ["SitePermissions", "resource:///modules/SitePermissions.jsm"],
   ["Social", "resource:///modules/Social.jsm"],
   ["TabCrashHandler", "resource:///modules/ContentCrashHandlers.jsm"],
   ["Task", "resource://gre/modules/Task.jsm"],
   ["TelemetryStopwatch", "resource://gre/modules/TelemetryStopwatch.jsm"],
   ["Translation", "resource:///modules/translation/Translation.jsm"],
   ["UITour", "resource:///modules/UITour.jsm"],
   ["UpdateUtils", "resource://gre/modules/UpdateUtils.jsm"],
+  ["URLBarZoom", "resource:///modules/URLBarZoom.jsm"],
   ["Weave", "resource://services-sync/main.js"],
   ["fxAccounts", "resource://gre/modules/FxAccounts.jsm"],
   ["gDevTools", "resource://devtools/client/framework/gDevTools.jsm"],
   ["gDevToolsBrowser", "resource://devtools/client/framework/gDevTools.jsm"],
-  ["webrtcUI", "resource:///modules/webrtcUI.jsm", ]
+  ["webrtcUI", "resource:///modules/webrtcUI.jsm"],
 ].forEach(([name, resource]) => XPCOMUtils.defineLazyModuleGetter(this, name, resource));
 
 XPCOMUtils.defineLazyModuleGetter(this, "SafeBrowsing",
   "resource://gre/modules/SafeBrowsing.jsm");
 
 if (AppConstants.MOZ_CRASHREPORTER) {
   XPCOMUtils.defineLazyModuleGetter(this, "PluginCrashReporter",
     "resource:///modules/ContentCrashHandlers.jsm");
@@ -1127,16 +1129,17 @@ var gBrowserInit = {
     LanguageDetectionListener.init();
     BrowserOnClick.init();
     FeedHandler.init();
     CompactTheme.init();
     AboutPrivateBrowsingListener.init();
     TrackingProtection.init();
     RefreshBlocker.init();
     CaptivePortalWatcher.init();
+    URLBarZoom.init(window);
 
     let mm = window.getGroupMessageManager("browsers");
     mm.loadFrameScript("chrome://browser/content/tab-content.js", true);
     mm.loadFrameScript("chrome://browser/content/content.js", true);
     mm.loadFrameScript("chrome://browser/content/content-UITour.js", true);
     mm.loadFrameScript("chrome://global/content/manifestMessages.js", true);
 
     // initialize observers and listeners

--- a/browser/components/nsBrowserGlue.js
+++ b/browser/components/nsBrowserGlue.js
@@ -27,17 +27,17 @@ XPCOMUtils.defineLazyServiceGetter(this,
           FileUtils:false, FormValidationHandler:false, Integration:false,
           LightweightThemeManager:false, LoginHelper:false, LoginManagerParent:false,
           NetUtil:false, NewTabMessages:false, NewTabUtils:false, OS:false,
           PageThumbs:false, PdfJs:false, PermissionUI:false, PlacesBackups:false,
           PlacesUtils:false, PluralForm:false, PrivateBrowsingUtils:false,
           ProcessHangMonitor:false, ReaderParent:false, RecentWindow:false,
           RemotePrompt:false, SelfSupportBackend:false, SessionStore:false,
           ShellService:false, SimpleServiceDiscovery:false, TabCrashHandler:false,
-          Task:false, UITour:false, URLBarZoom:false, WebChannel:false,
+          Task:false, UITour:false, WebChannel:false,
           WindowsRegistry:false, webrtcUI:false */
 
 /**
  * IF YOU ADD OR REMOVE FROM THIS LIST, PLEASE UPDATE THE LIST ABOVE AS WELL.
  * XXX Bug 1325373 is for making eslint detect these automatically.
  */
 
 [
@@ -81,17 +81,16 @@ XPCOMUtils.defineLazyServiceGetter(this,
   ["RemotePrompt", "resource:///modules/RemotePrompt.jsm"],
   ["SelfSupportBackend", "resource:///modules/SelfSupportBackend.jsm"],
   ["SessionStore", "resource:///modules/sessionstore/SessionStore.jsm"],
   ["ShellService", "resource:///modules/ShellService.jsm"],
   ["SimpleServiceDiscovery", "resource://gre/modules/SimpleServiceDiscovery.jsm"],
   ["TabCrashHandler", "resource:///modules/ContentCrashHandlers.jsm"],
   ["Task", "resource://gre/modules/Task.jsm"],
   ["UITour", "resource:///modules/UITour.jsm"],
-  ["URLBarZoom", "resource:///modules/URLBarZoom.jsm"],
   ["WebChannel", "resource://gre/modules/WebChannel.jsm"],
   ["WindowsRegistry", "resource://gre/modules/WindowsRegistry.jsm"],
   ["webrtcUI", "resource:///modules/webrtcUI.jsm"],
 ].forEach(([name, resource]) => XPCOMUtils.defineLazyModuleGetter(this, name, resource));
 
 if (AppConstants.MOZ_CRASHREPORTER) {
   XPCOMUtils.defineLazyModuleGetter(this, "PluginCrashReporter",
                                     "resource:///modules/ContentCrashHandlers.jsm");
@@ -626,17 +625,16 @@ BrowserGlue.prototype = {
 
     ContentClick.init();
     RemotePrompt.init();
     Feeds.init();
     ContentPrefServiceParent.init();
 
     LoginManagerParent.init();
     ReaderParent.init();
-    URLBarZoom.init();
 
     SelfSupportBackend.init();
 
     // Ensure we keep track of places/pw-mananager undo by init'ing this early.
     Cu.import("resource:///modules/AutoMigrate.jsm");
 
     if (AppConstants.INSTALL_COMPACT_THEMES) {
       let vendorShortName = gBrandBundle.GetStringFromName("vendorShortName");

--- a/browser/modules/URLBarZoom.jsm
+++ b/browser/modules/URLBarZoom.jsm
@@ -5,47 +5,71 @@
 
 "use strict";
 
 this.EXPORTED_SYMBOLS = [ "URLBarZoom" ];
 
 Components.utils.import("resource://gre/modules/Services.jsm");
 
 var URLBarZoom = {
-
   init(aWindow) {
-    // Register ourselves with the service so we know when the zoom prefs change.
-    Services.obs.addObserver(updateZoomButton, "browser-fullZoom:zoomChange", false);
-    Services.obs.addObserver(updateZoomButton, "browser-fullZoom:zoomReset", false);
-    Services.obs.addObserver(updateZoomButton, "browser-fullZoom:location-change", false);
+    aWindow.addEventListener("EndSwapDocShells", onEndSwapDocShells, true);
+    aWindow.addEventListener("unload", () => {
+      aWindow.removeEventListener("EndSwapDocShells", onEndSwapDocShells, true);
+    }, {once: true});
   },
 }
 
-function updateZoomButton(aSubject, aTopic) {
-  let win = aSubject.ownerGlobal;
+function fullZoomObserver(aSubject, aTopic) {
+  // If the tab was the last one in its window and has been dragged to another
+  // window, the original browser's window will be unavailable here. Since that
+  // window is closing, we can just ignore this notification.
+  if (!aSubject.ownerGlobal) {
+    return;
+  }
+
+  // Only allow pulse animation for zoom changes, not tab switching.
+  let animate = (aTopic != "browser-fullZoom:location-change");
+  updateZoomButton(aSubject, animate);
+}
+
+function onEndSwapDocShells(event) {
+  updateZoomButton(event.originalTarget);
+}
+
+function updateZoomButton(aBrowser, aAnimate = false) {
+  let win = aBrowser.ownerGlobal;
+  if (aBrowser != win.gBrowser.selectedBrowser) {
+    return;
+  }
+
   let customizableZoomControls = win.document.getElementById("zoom-controls");
   let zoomResetButton = win.document.getElementById("urlbar-zoom-button");
-  let zoomFactor = Math.round(win.ZoomManager.zoom * 100);
 
   // Ensure that zoom controls haven't already been added to browser in Customize Mode
   if (customizableZoomControls &&
       customizableZoomControls.getAttribute("cui-areatype") == "toolbar") {
     zoomResetButton.hidden = true;
     return;
   }
+
+  let zoomFactor = Math.round(win.ZoomManager.zoom * 100);
   if (zoomFactor != 100) {
     // Check if zoom button is visible and update label if it is
     if (zoomResetButton.hidden) {
       zoomResetButton.hidden = false;
     }
-    // Only allow pulse animation for zoom changes, not tab switching
-    if (aTopic != "browser-fullZoom:location-change") {
+    if (aAnimate) {
       zoomResetButton.setAttribute("animate", "true");
     } else {
       zoomResetButton.removeAttribute("animate");
     }
     zoomResetButton.setAttribute("label",
         win.gNavigatorBundle.getFormattedString("urlbar-zoom-button.label", [zoomFactor]));
-  // Hide button if zoom is at 100%
   } else {
-      zoomResetButton.hidden = true;
+    // Hide button if zoom is at 100%
+    zoomResetButton.hidden = true;
   }
 }
+
+Services.obs.addObserver(fullZoomObserver, "browser-fullZoom:zoomChange", false);
+Services.obs.addObserver(fullZoomObserver, "browser-fullZoom:zoomReset", false);
+Services.obs.addObserver(fullZoomObserver, "browser-fullZoom:location-change", false);