Aha. Looks like the problem might be that your web server is sending it as text/html (and the filename is .html) and Mozilla wants it to be application/xhtml+xml or text/xml or application/xml (and .xhtml) in order for it to be passed through its xhtml renderer.

I can't really test it right now, since it depends on other things on your site to render properly, but try seeing if you can fix that, and I'll give it another go.