diff --git a/dom/indexedDB/test/browser_permissionsPrompt.html b/dom/indexedDB/test/browser_permissionsPrompt.html
index 37c55775cb00..0ee698b97bf2 100644
--- a/dom/indexedDB/test/browser_permissionsPrompt.html
+++ b/dom/indexedDB/test/browser_permissionsPrompt.html
@@ -19,7 +19,7 @@
         let event = yield undefined;
 
         if (event.type == "success") {
-          testResult = event.target.result;
+          testResult = event.target.result instanceof IDBDatabase;
         }
         else {
           testException = event.target.error.name;
diff --git a/dom/indexedDB/test/browser_permissionsPromptAllow.js b/dom/indexedDB/test/browser_permissionsPromptAllow.js
index 577ad252862c..dd0921872571 100644
--- a/dom/indexedDB/test/browser_permissionsPromptAllow.js
+++ b/dom/indexedDB/test/browser_permissionsPromptAllow.js
@@ -24,8 +24,8 @@ function test1()
   gBrowser.selectedBrowser.addEventListener("load", function () {
     gBrowser.selectedBrowser.removeEventListener("load", arguments.callee, true);
 
-    setFinishedCallback(function(result, exception) {
-      ok(result instanceof IDBDatabase,
+    setFinishedCallback(function(isIDBDatabase, exception) {
+      ok(isIDBDatabase,
          "First database creation was successful");
       ok(!exception, "No exception");
       is(getPermission(testPageURL, "indexedDB"),
@@ -60,8 +60,8 @@ function test2()
   gBrowser.selectedBrowser.addEventListener("load", function () {
     gBrowser.selectedBrowser.removeEventListener("load", arguments.callee, true);
 
-    setFinishedCallback(function(result, exception) {
-      ok(result instanceof IDBDatabase,
+    setFinishedCallback(function(isIDBDatabase, exception) {
+      ok(isIDBDatabase,
          "First database creation was successful");
       ok(!exception, "No exception");
       is(getPermission(testPageURL, "indexedDB"),
diff --git a/dom/indexedDB/test/browser_perwindow_privateBrowsing.js b/dom/indexedDB/test/browser_perwindow_privateBrowsing.js
index e872a0650b1e..08d329cbc4c6 100644
--- a/dom/indexedDB/test/browser_perwindow_privateBrowsing.js
+++ b/dom/indexedDB/test/browser_perwindow_privateBrowsing.js
@@ -25,8 +25,8 @@ function test1()
     }
     gBrowser.selectedBrowser.removeEventListener("load", arguments.callee, true);
 
-    setFinishedCallback(function(result, exception) {
-      ok(result instanceof IDBDatabase,
+    setFinishedCallback(function(isIDBDatabase, exception) {
+      ok(isIDBDatabase,
          "First database creation was successful");
       ok(!exception, "No exception");
       gBrowser.removeCurrentTab();
@@ -57,8 +57,8 @@ function test3(win)
     }
     win.gBrowser.selectedBrowser.removeEventListener("load", arguments.callee, true);
 
-    setFinishedCallback(function(result, exception) {
-      ok(!result, "No database");
+    setFinishedCallback(function(isIDBDatabase, exception) {
+      ok(!isIDBDatabase, "No database");
       is(exception, "InvalidStateError", "Correct exception");
       win.gBrowser.removeCurrentTab();
 
diff --git a/dom/indexedDB/test/head.js b/dom/indexedDB/test/head.js
index 9021c3e2c3a8..898a40e8f7b3 100644
--- a/dom/indexedDB/test/head.js
+++ b/dom/indexedDB/test/head.js
@@ -92,13 +92,16 @@ function setFinishedCallback(callback, win)
   if (!win) {
     win = window;
   }
-  let testPage = win.gBrowser.selectedBrowser.contentWindow.wrappedJSObject;
-  testPage.testFinishedCallback = function(result, exception) {
-    setTimeout(function() {
-      info("got finished callback");
-      callback(result, exception);
-    }, 0);
-  }
+  ContentTask.spawn(win.gBrowser.selectedBrowser, null, function*() {
+    return yield new Promise(resolve => {
+      content.wrappedJSObject.testFinishedCallback = (result, exception) => {
+        info("got finished callback");
+        resolve({result, exception});
+      };
+    });
+  }).then(({result, exception}) => {
+    callback(result, exception);
+  });
 }
 
 function dispatchEvent(eventName)