feat(firefox): support running beforeunload hooks when closing (#4003)

test/assets/beforeunload.html

@@ -1,5 +1,10 @@
+<div>beforeunload demo.</div>
 <script>
 window.addEventListener('beforeunload', event => {
+  // Chrome way.
   event.returnValue = 'Leave?';
+  // Firefox way.
+  event.preventDefault();
 });
 </script>
+