95% of the time I use Firefox, and I love it to death.
Occasionally I'll use the old Mozilla Phoenix for certain sites. I have some sites bookmarked with that one or with user/passes stored on that one so that's why I'd use it. That's because I used Phx for awhile before switching to Firebird, and then to Firefox.
I pretty much only use IE for Yahoo! Launchcast Plus and to check browser compatibility. Or do Windows updates or view sites that are IE-dependant.