setIntervalOnBeforeUnloadBug.html

<script>
window.onbeforeunload = function() {
  return "confirm";
}

var i = 0;
setInterval(function() {
  document.getElementById('display').innerHTML = i++;
}, 100);

</script>

<h1>Demonstration of a Firefox bug</h1>

<ol>
  <li>Hit F5 to refresh the page, but do not click anything on the confirmation dialog. <em>The timer will have stopped, but this is probably intended.</em></li>
  <li>Change to a different tab and back. <em>The dialog will be gone</em></li>
  <li>Hit F5 again, but confirm the reload this time</li>
  <li><em>The timer will not start again, even after pressing Ctrl+F5</em></li>
</ol>

<div id="display"></div>