--- a/browser/components/customizableui/test/browser_panelUINotifications_fullscreen_noAutoHideToolbar.js
+++ b/browser/components/customizableui/test/browser_panelUINotifications_fullscreen_noAutoHideToolbar.js
@@ -36,16 +36,17 @@ add_task(async function testFullscreen()
   }
 
   is(PanelUI.notificationPanel.state, "closed", "update-manual doorhanger is closed.");
   let mainActionCalled = false;
   let mainAction = {
     callback: () => { mainActionCalled = true; }
   };
   AppMenuNotifications.showNotification("update-manual", mainAction);
+  await BrowserTestUtils.waitForEvent(PanelUI.notificationPanel, "popupshown");
 
   isnot(PanelUI.notificationPanel.state, "closed", "update-manual doorhanger is showing.");
   let notifications = [...PanelUI.notificationPanel.children].filter(n => !n.hidden);
   is(notifications.length, 1, "PanelUI doorhanger is only displaying one notification.");
   let doorhanger = notifications[0];
   is(doorhanger.id, "appMenu-update-manual-notification", "PanelUI is displaying the update-manual notification.");
 
   let fullscreenPromise = waitForFullscreen();