diff --git a/accessible/tests/mochitest/browser.js b/accessible/tests/mochitest/browser.js
index a396cd26b3bd..74256d434059 100644
--- a/accessible/tests/mochitest/browser.js
+++ b/accessible/tests/mochitest/browser.js
@@ -135,6 +135,11 @@ function openBrowserWindowIntl() {
 
 function startBrowserTests() {
   if (gBrowserContext.startURL) {
+    // Make sure the window is the one loading our URL.
+    if (currentBrowser().contentWindow.location != gBrowserContext.startURL) {
+      setTimeout(startBrowserTests, 0);
+      return;
+    }
     // wait for load
     addA11yLoadEvent(gBrowserContext.testFunc, currentBrowser().contentWindow);
   } else {