--- a/browser/base/content/browser-addons.js
+++ b/browser/base/content/browser-addons.js
@@ -491,47 +491,66 @@ var LightWeightThemeWebInstaller = {
 
   _installRequest: function (dataString, baseURI) {
     let data = this._manager.parseTheme(dataString, baseURI);
 
     if (!data) {
       return;
     }
 
+    let uri = makeURI(baseURI);
+
+    // A notification bar with the option to undo is normally shown after a
+    // theme is installed.  But the discovery pane served from the url(s)
+    // below has its own toggle switch for quick undos, so don't show the
+    // notification in that case.
+    let notify = uri.prePath != "https://discovery.addons.mozilla.org";
+    if (notify) {
+      try {
+        if (Services.prefs.getBoolPref("extensions.webapi.testing")
+            && (uri.prePath == "https://discovery.addons.allizom.org"
+                || uri.prePath == "https://discovery.addons-dev.allizom.org")) {
+          notify = false;
+        }
+      } catch (e) {
+        // getBoolPref() throws if the testing pref isn't set.  ignore it.
+      }
+    }
+
     if (this._isAllowed(baseURI)) {
-      this._install(data);
+      this._install(data, notify);
       return;
     }
 
     let allowButtonText =
       gNavigatorBundle.getString("lwthemeInstallRequest.allowButton");
     let allowButtonAccesskey =
       gNavigatorBundle.getString("lwthemeInstallRequest.allowButton.accesskey");
     let message =
       gNavigatorBundle.getFormattedString("lwthemeInstallRequest.message",
-                                          [makeURI(baseURI).host]);
+                                          [uri.host]);
     let buttons = [{
       label: allowButtonText,
       accessKey: allowButtonAccesskey,
       callback: function () {
-        LightWeightThemeWebInstaller._install(data);
+        LightWeightThemeWebInstaller._install(data, notify);
       }
     }];
 
     this._removePreviousNotifications();
 
     let notificationBox = gBrowser.getNotificationBox();
     let notificationBar =
       notificationBox.appendNotification(message, "lwtheme-install-request", "",
                                          notificationBox.PRIORITY_INFO_MEDIUM,
                                          buttons);
     notificationBar.persistence = 1;
   },
 
-  _install: function (newLWTheme) {
+  _install: function (newLWTheme, notify) {
     let previousLWTheme = this._manager.currentTheme;
 
     let listener = {
       onEnabling: function(aAddon, aRequiresRestart) {
         if (!aRequiresRestart) {
           return;
         }
 
@@ -551,17 +570,19 @@ var LightWeightThemeWebInstaller = {
         };
 
         PopupNotifications.show(gBrowser.selectedBrowser, "addon-theme-change",
                                 messageString, "addons-notification-icon",
                                 action, null, options);
       },
 
       onEnabled: function(aAddon) {
-        LightWeightThemeWebInstaller._postInstallNotification(newLWTheme, previousLWTheme);
+        if (notify) {
+          LightWeightThemeWebInstaller._postInstallNotification(newLWTheme, previousLWTheme);
+        }
       }
     };
 
     AddonManager.addAddonListener(listener);
     this._manager.currentTheme = newLWTheme;
     AddonManager.removeAddonListener(listener);
   },