Firefox all the way. I just use IE to test sites to make sure they work. I use IE at work because some of our internal sites don't work with Firefox
Firefox, I'm so glad I switched to it from IE when I did, it's so much better. When I use IE for something now I get so p*ssed off with it
I think Firefox runs a lot smoother than the rest, however there are times when certain web applications don't run on it...so when that time comes I switch over to IE for a couple seconds.
I'm impressed so little people like opera. The only thing that sets them back is that the browser doesn't come with a decent spell check function like FF. The day they get that right, bye bye FF.