Actually the site doesn't look good in FF. It looks like some elements are out of place.

The W3C xhtml validator has 83 errors and the css won't validate at all because the xhtml won't validate but I can tell you from looking at it with my Firefox tools that there is a shedload of css errors too.

I think fixing these is the place to start.

Oh and if you are going to the trouble of converting to xhtml, get rid of all those table bits as layout devices in favour of div's.

If your going to do it, do it properly.