Great but last time I tried it was too buggy for me. Specifically it had problems filling in forms and with script pages. Scripts that took too long to run it wouldn't process and any page with image recognition on (such as most TGP submit pages) didn't work at all.
It could be these have been sorted now but until I'm certain they have I'll keep clear of it. The script timeout thing was very, very annoying.
I don't mind admitting to using IE simply because most of my surfers do and 80% of my day is spent changing and building pages. Even with plugins available in FF it's still easier to simply use the browser most of your punters use. IE6 is stable and secure enough (if kept updated).
I often use FF but usually when doing stuff where the plugins become handy
(thought I'd add all that before the idiots come in shouting about how great FF is, how dumb I am to use IE at all and how cool they are to use FF because MS doesn't own it). Horses for courses so to speak.