--- a/browser/base/content/test/sync/browser_sync.js
+++ b/browser/base/content/test/sync/browser_sync.js
@@ -1,13 +1,18 @@
 /* Any copyright is dedicated to the Public Domain.
    http://creativecommons.org/publicdomain/zero/1.0/ */
 
 "use strict";
 
+add_task(function setup() {
+  // gSync.init() is called in a requestIdleCallback. Force its initialization.
+  gSync.init();
+});
+
 add_task(async function test_ui_state_notification_calls_updateAllUI() {
   let called = false;
   let updateAllUI = gSync.updateAllUI;
   gSync.updateAllUI = () => { called = true; };
 
   Services.obs.notifyObservers(null, UIState.ON_UPDATE);
   ok(called);