I've been using Firefox for a year and it is my preferred browser. However, functionality is reduced on a few sites that cater specifically for Microsoft software. For instance, on this site, the 'Recent Posts' pod does not refresh when you click the 'refresh' button - you have to refresh the whole page. Posting to MSN groups is also restricted where you do not have access to the toolbar or HTML editing (posting images it therefore not possible with Firefox).
PS
I have not yet upgraded to Firefox 3.0 so some of those comments may no longer apply.