Where can I find an XSD/DTD for XUL which WebStorm will like?

Question

I'm trying to use JetBrains' WebStorm IDE to work on a Mozilla Thunderbird extension of mine. It's not giving me such an easy time... but the worst part is all the error messages about my XUL files.

If I go to File | Settings | Languages and Frameworks | Schemas and DTDs - I'm supposed to be able to enter links/paths to some files with definitions of my elements, so that WebStorm won't balk at them. But - where do I find a DTD or an XSD for XUL, with elements such as dialog, script, hbox, vbox etc.?

Here's a slimmed-down version of my XUL file (only a few of the elements), to give you an idea of what I'm talking about :

<?xml version="1.0"?>

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

<!DOCTYPE dialog SYSTEM "chrome://foo/locale/removedupes.dtd">

<dialog xmlns="http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul"
        persist="width height screenX screenY sizemode"
        buttons="accept,cancel"
        onload="someFunction();"
        id="foo_dialog"
        title="&foo.dialog_title;">

  <script type="application/javascript" src="chrome://global/content/globalOverlay.js"/>

  <hbox flex="1">
    <vbox flex="2">
      <description>&foo.some_label;</description>
      <spacer flex="1"/>
      </vbox>
    </hbox>
  </hbox>
</dialog>