I use Firefox regularly, and would say that in general it is more stable and faster. Although the recent release of Firefox 3 was not very good for me at all, and included several bugs... although these have now been fixed so I am back with Firefox.
Firefox as well, is better, from a platform compatibility perspective. This is made obvious by the fact that Firefox support OS X (Mac) where as IE stopped supporting Mac years ago. They even took the download off their site [although
some places you can still get it] but not officially anymore.
Taking everything into consideration I would probably say Firefox is better