http://stackoverflow.com/questions/6631637/how-to-get-reference-to-xul-iframe-that-contains-a-window , http://stackoverflow.com/questions/11174359/how-to-properly-receive-the-domcontentloaded-event-from-an-xul-iframe

stdin.xul

<?xml version="1.0" encoding="UTF-8"?>
<?xml-stylesheet href="chrome://global/skin/" type="text/css"?>

<window id="foo" orient="vertical"
      xmlns="http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul">

  <script type="application/javascript"><![CDATA[
    onload = function onload(event) {
      let iframe = document.getElementById("foo-iframe");
      iframe.addEventListener("DOMContentLoaded", function (event) {
        alert("You'll never see me - but no one knows why...");
        if (this != event.originalTarget)
          return;
        // doStuffWith(this.contentWindow);
      }, true);
      iframe.setAttribute("src", "http://www.google.com/");
    }
  ]]></script>

  <iframe id="foo-iframe" type="content"/>
</window>

This is all I see:

(No alert pop-up.)