--- a/toolkit/components/extensions/test/mochitest/test_ext_contentscript_about_blank.html
+++ b/toolkit/components/extensions/test/mochitest/test_ext_contentscript_about_blank.html
@@ -64,23 +64,31 @@ add_task(function* test_contentscript_ab
       browser.test.sendMessage(`${script}:${kind}`);
     });
   }
 
   const PATH = "tests/toolkit/components/extensions/test/mochitest/file_with_about_blank.html";
   const extension = ExtensionTestUtils.loadExtension({manifest, files, background});
   yield extension.startup();
 
+  let win;
+  SimpleTest.registerCleanupFunction(() => {
+    if (win && !win.closed) {
+      win.close();
+    }
+    return extension.unload();
+  });
+
   let count = 0;
   extension.onMessage("script", script => {
     info(`script ran: ${script}`);
     count++;
   });
 
-  let win = window.open("http://example.com/" + PATH);
+  win = window.open("http://example.com/" + PATH);
   yield Promise.all([
     extension.awaitMessage("all:top"),
     extension.awaitMessage("all:about:blank"),
     extension.awaitMessage("all:about:srcdoc"),
   ]);
   is(count, 3, "exactly 3 scripts ran");
   win.close();
 
@@ -103,15 +111,13 @@ add_task(function* test_contentscript_ab
   let a_b = win.document.getElementById("a_b");
   style = a_b.contentWindow.getComputedStyle(a_b.contentDocument.body);
   is(style.color, "rgb(255, 0, 0)", "about:blank iframe text color is red");
   is(style.backgroundColor, "transparent", "about:blank iframe background is transparent");
   is(style.textAlign, "right", "about:blank text is right-aligned");
 
   is(count, 10, "exactly 7 more scripts ran");
   win.close();
-
-  yield extension.unload();
 });
 </script>
 
 </body>
 </html>