--- a/dom/browser-element/mochitest/browserElement_ContextmenuEvents.js
+++ b/dom/browser-element/mochitest/browserElement_ContextmenuEvents.js
@@ -110,17 +110,23 @@ function runTest() {
     } else if (ctxCallbackEvents === 2) {
       ok(msg.json.data === 'foo', 'Callback function got fired correctly');
       mm.loadFrameScript('data:,(' + trigger1.toString() + ')();', false);
     } else if (ctxCallbackEvents > 2) {
       ok(false, 'Too many callback events');
     }
   }
 
+  var gotError = false;
   function errorTriggered(msg) {
+    if (gotError) {
+      return;
+    }
+
+    gotError = true;
     ok(true, 'An error in the callback triggers window.onerror');
     SimpleTest.finish();
   }
 
   function iframeLoadedHandler() {
     numIframeLoaded++;
     if (numIframeLoaded === 2) {
       mm = SpecialPowers.getBrowserFrameMessageManager(iframe1);