Since upgrading to WordPress 3.1, I've dound that I have to disable the XHTML content negotiation plugin that I was using, otherwise the pages fail to render in Firefox, coming ups with an error that says:
XML Parsing Error: not well-formed... & then lists the line causing the problem (somewhere near the end of the pages code).
As far as I can work out from having a quick look at the page source, this appears to be part of the code, that now creates the bar at the top of the page - in particular the part showing the site visitor stats.
So its not really a problem with the XHTML content negotiation plugin - but more that running this highlights the badly formed XHTML code that is produced by WordPress / The site stats plugin.
Has anyone else come across this problem? I tried fixing it by dropping from XHTML 1.1 to 1.0 transitional, but it seems that disabling the content negotiation plugin is the only way of making the pages render in Firefox.