--- a/browser/components/nsBrowserGlue.js
+++ b/browser/components/nsBrowserGlue.js
@@ -1774,17 +1774,17 @@ BrowserGlue.prototype = {
       if (topic != "alertclickcallback")
         return;
       this._openPreferences("sync");
     }
     AlertsService.showAlertNotification(null, title, body, true, null, clickCallback);
   },
 
   _migrateUI: function BG__migrateUI() {
-    const UI_VERSION = 40;
+    const UI_VERSION = 41;
     const BROWSER_DOCURL = "chrome://browser/content/browser.xul";
 
     let currentUIVersion;
     if (Services.prefs.prefHasUserValue("browser.migration.version")) {
       currentUIVersion = Services.prefs.getIntPref("browser.migration.version");
     } else {
       // This is a new profile, nothing to migrate.
       Services.prefs.setIntPref("browser.migration.version", UI_VERSION);
@@ -2100,16 +2100,21 @@ BrowserGlue.prototype = {
         Services.prefs.setBoolPref("browser.safebrowsing.phishing.enabled",
                                    false);
         // Should just remove support for the pref entirely, even if it's
         // only in about:config
         Services.prefs.clearUserPref(kOldSafeBrowsingPref);
       }
     }
 
+    if (currentUIVersion < 41) {
+      const Preferences = Cu.import("resource://gre/modules/Preferences.jsm", {}).Preferences;
+      Preferences.resetBranch("loop.");
+    }
+
     // Update the migration version.
     Services.prefs.setIntPref("browser.migration.version", UI_VERSION);
   },
 
   _hasExistingNotificationPermission: function BG__hasExistingNotificationPermission() {
     let enumerator = Services.perms.enumerator;
     while (enumerator.hasMoreElements()) {
       let permission = enumerator.getNext().QueryInterface(Ci.nsIPermission);

new file mode 100644
--- /dev/null
+++ b/browser/components/tests/unit/head.js
@@ -0,0 +1,9 @@
+/* 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 {interfaces: Ci, classes: Cc, results: Cr, utils: Cu} = Components;
+
+Cu.import("resource://gre/modules/Services.jsm");
+
+var gProfD = do_get_profile().QueryInterface(Ci.nsILocalFile);

new file mode 100644
--- /dev/null
+++ b/browser/components/tests/unit/test_browserGlue_migration_loop_cleanup.js
@@ -0,0 +1,32 @@
+const UI_VERSION = 41;
+const TOPIC_BROWSERGLUE_TEST = "browser-glue-test";
+const TOPICDATA_BROWSERGLUE_TEST = "force-ui-migration";
+
+var gBrowserGlue = Cc["@mozilla.org/browser/browserglue;1"]
+                     .getService(Ci.nsIObserver);
+
+Services.prefs.setIntPref("browser.migration.version", UI_VERSION - 1);
+
+add_task(function* test_check_cleanup_loop_prefs() {
+  Services.prefs.setBoolPref("loop.createdRoom", true);
+  Services.prefs.setBoolPref("loop1.createdRoom", true);
+  Services.prefs.setBoolPref("loo.createdRoom", true);
+
+  // Simulate a migration.
+  gBrowserGlue.observe(null, TOPIC_BROWSERGLUE_TEST, TOPICDATA_BROWSERGLUE_TEST);
+
+  Assert.throws(() => Services.prefs.getBoolPref("loop.createdRoom"),
+                /NS_ERROR_UNEXPECTED/,
+                "should have cleared old loop preference 'loop.createdRoom'");
+  Assert.ok(Services.prefs.getBoolPref("loop1.createdRoom"),
+            "should have left non-loop pref 'loop1.createdRoom' untouched");
+  Assert.ok(Services.prefs.getBoolPref("loo.createdRoom"),
+            "should have left non-loop pref 'loo.createdRoom' untouched");
+});
+
+do_register_cleanup(() => {
+  Services.prefs.clearUserPref("browser.migration.version");
+  Services.prefs.clearUserPref("loop.createdRoom");
+  Services.prefs.clearUserPref("loop1.createdRoom");
+  Services.prefs.clearUserPref("loo.createdRoom");
+});

--- a/browser/components/tests/unit/test_distribution.js
+++ b/browser/components/tests/unit/test_distribution.js
@@ -1,21 +1,15 @@
 /* Any copyright is dedicated to the Public Domain.
    http://creativecommons.org/publicdomain/zero/1.0/ */
 
 /**
  * Tests that preferences are properly set by distribution.ini
  */
 
-var Ci = Components.interfaces;
-var Cc = Components.classes;
-var Cr = Components.results;
-var Cu = Components.utils;
-
-Cu.import("resource://gre/modules/Services.jsm");
 Cu.import("resource://gre/modules/LoadContextInfo.jsm");
 
 // Import common head.
 var commonFile = do_get_file("../../../../toolkit/components/places/tests/head_common.js", false);
 if (commonFile) {
   let uri = Services.io.newFileURI(commonFile);
   Services.scriptloader.loadSubScript(uri.spec, this);
 }
@@ -26,18 +20,16 @@ const TOPIC_BROWSERGLUE_TEST = "browser-
 /**
  * Copy the engine-distribution.xml engine to a fake distribution
  * created in the profile, and registered with the directory service.
  * Create an empty en-US directory to make sure it isn't used.
  */
 function installDistributionEngine() {
   const XRE_APP_DISTRIBUTION_DIR = "XREAppDist";
 
-  const gProfD = do_get_profile().QueryInterface(Ci.nsILocalFile);
-
   let dir = gProfD.clone();
   dir.append("distribution");
   let distDir = dir.clone();
 
   dir.append("searchplugins");
   dir.create(dir.DIRECTORY_TYPE, FileUtils.PERMS_DIRECTORY);
 
   dir.append("locale");

--- a/browser/components/tests/unit/xpcshell.ini
+++ b/browser/components/tests/unit/xpcshell.ini
@@ -1,8 +1,10 @@
 [DEFAULT]
+head = head.js
 firefox-appdir = browser
 skip-if = toolkit == 'android' || toolkit == 'gonk'
 support-files =
   distribution.ini
   data/engine-de-DE.xml
 
 [test_distribution.js]
+[test_browserGlue_migration_loop_cleanup.js]