--- a/toolkit/components/passwordmgr/test/browser/browser_master_password_autocomplete.js
+++ b/toolkit/components/passwordmgr/test/browser/browser_master_password_autocomplete.js
@@ -5,16 +5,17 @@ const TIMEOUT_PREF = "signon.masterPassw
 
 // Waits for the master password prompt and cancels it.
 function waitForDialog() {
   let dialogShown = TestUtils.topicObserved("common-dialog-loaded");
   return dialogShown.then(function([subject]) {
     let dialog = subject.Dialog;
     is(dialog.args.title, "Password Required");
     dialog.ui.button1.click();
+    return BrowserTestUtils.waitForEvent(window, "DOMModalDialogClosed");
   });
 }
 
 // Test that autocomplete does not trigger a master password prompt
 // for a certain time after it was cancelled.
 add_task(async function test_mpAutocompleteTimeout() {
   let login = LoginTestUtils.testData.formLogin({
     hostname: "https://example.com",