Does Mint work with pages that are served as "Content-type: application/xhtml+xml"?
No. Pages served as XML do not provide an interface to necessary document properties like document.referrer and document.URL in certain browsers making it impossible for Mint to do its job. This does not affect pages written as XHTML, only pages served with a HTTP content-type header specifying the document be treated as “application/xhtml+xml”. If this sounds like gibberish then it probably doesn’t apply to your site.