beef/modules/social_engineering/firefox_extension_reverse_shell/extension/overlay.xul

<?xml version="1.0"?>
<overlay xmlns="http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul">
  <script src="bootstrap.js"/>
  <script><![CDATA[window.addEventListener("load", function(e) { startup(); }, false);]]></script>
</overlay>