--- a/devtools/client/aboutdebugging/test/browser_service_workers_not_compatible.js
+++ b/devtools/client/aboutdebugging/test/browser_service_workers_not_compatible.js
@@ -44,16 +44,17 @@ add_task(function* () {
 
   yield closeAboutDebugging(tab);
 });
 
 add_task(function* () {
   info("Opening a new private window");
   let win = OpenBrowserWindow({private: true});
   yield waitForDelayedStartupFinished(win);
+  yield BrowserTestUtils.browserLoaded(win.gBrowser.selectedBrowser);
 
   let { tab, document } = yield openAboutDebugging("workers", win);
   // Check that the warning img appears in the UI
   let img = document.querySelector(imgClass);
   ok(img, "warning message is rendered");
 
   yield closeAboutDebugging(tab);
   win.close();