Original DOM of a XSLT'd XML

Hi,

I'm testing for certain types of XML documents when a page loads in the
xul:browser.  But if an XML document has an XSLT stylesheet associated
with it (feedburner feeds for instance) then the browser document is
the DOM for the transform result but not the original XML document.
How can I get hold of the original?  I'd be happy even if I could get
it as a string which I could maybe test with a regex and then parse
with DOMParser if I needed to.

I don't want to do XMLDocument.load() everytime a page loads just to
see if it really is the XML I want.  XMLDocument.load probably doesn't
even use the browser cache, does it?  And I don't want to do
nsIWebBrowserPersist.saveURI() to a local file either.  Is there any
way to get at the in memory DOM of the original XML doc or even its xml
string?

Thanks for your help