_run.sh

./mach run --setpref dom.allow_XUL_XBL_for_file=true file:///path/to/minimal.xul

minimal.xul

<?xml version="1.0"?>
<!-- This Source Code Form is subject to the terms of the Mozilla Public
   - License, v. 2.0. If a copy of the MPL was not distributed with this file,
   - You can obtain one at http://mozilla.org/MPL/2.0/. -->

<?xml-stylesheet href="chrome://global/skin/global.css"?>

<page xmlns="http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul"
      xmlns:html="http://www.w3.org/1999/xhtml"
      title="Test">
  <box>Hi</box>

  <script type="application/javascript"><![CDATA[
  alert("hi");
  ]]></script>
</page>