diff --git a/toolkit/components/passwordmgr/test/mochitest/test_autocomplete_tab_between_fields.html b/toolkit/components/passwordmgr/test/mochitest/test_autocomplete_tab_between_fields.html
--- a/toolkit/components/passwordmgr/test/mochitest/test_autocomplete_tab_between_fields.html
+++ b/toolkit/components/passwordmgr/test/mochitest/test_autocomplete_tab_between_fields.html
@@ -93,11 +93,11 @@
 add_setup(async () => {
   await SpecialPowers.pushPrefEnv({"set": [["signon.includeOtherSubdomainsInLookup", true]]});
 });
 
 async function testResultOfTabInteractions(testData) {
-  const logins = testData.logins.map(name => availableLogins[name]);
+  let logins = testData.logins.map(name => availableLogins[name]);
   await setStoredLoginsAsync(...logins);
 
   const form = createLoginForm({
     action: "https://autofill"
   });
@@ -110,19 +110,21 @@
 
   SpecialPowers.wrap(form.pword).setUserInput("");
   SpecialPowers.wrap(form.uname).setUserInput("");
 
   info("Placing focus in the password field");
+  const shownPromise = popupBy();
   form.pword.focus();
   await synthesizeKey("KEY_Tab", { shiftKey: true }); // blur pw, focus un
   await new Promise(resolve => SimpleTest.executeSoon(resolve));
 
   // moving focus shouldn't change anything
   checkForm(1, "", "");
 
+  await synthesizeKey("KEY_ArrowDown");
   info("waiting for AC results");
-  const results = await popupByArrowDown();
+  let results = await shownPromise;
   info("checking results");
   checkAutoCompleteResults(results, testData.expectedACLabels,
                            window.location.host, "Check all rows are correct");
 
   if (testData.typeUsername) {
@@ -149,12 +151,12 @@
 
   ok(form.pword.matches("input:focus"), "pword field is focused");
   checkForm(1, testData.expectedTabbedUsername, testData.expectedTabbedPassword);
 }
 
-for (const testData of tests) {
-  const tmp = {
+for (let testData of tests) {
+  let tmp = {
     async [testData.name]() {
       await testResultOfTabInteractions(testData);
     },
   };
   add_task(tmp[testData.name]);