Firefox is a great browser, the only thing it doesn't do is to deal with .Net enabled applications well. So if you, as I do, have to deal with Sharepoint sites on a daily basis (or other .Net enabled applications), you may find that some things just don't work.
Always the joy with "other" party apps and Microsoft isn't it?
If you are looking at Firefox, look at Thunderbird as well, a darned nice email package.
I've been using it for over a year now and the number of sites that don't render correctly has dropped quite a bit. The only time I ever have to resort to IE is one or two sites that stubbornly refuse to change.
I have been using Mozilla since quite some time now. I just love the tab feature. I have several groups set up (personal, work).
At times there are scripts that make me use IE, but that happens rarely.