I use them both, but prefer FF. FF is more secure, updated much faster, tons of addons for custimzation, a MUCH better search feature (automatically searches as you type, and seaches the WHOLE page, not just what's above or below your mouse curser), and USED to have less overhead, but I checked it recently and at least on my machine used slightly more memory than IE. Also, unlike IE 7 you can actually drag a tab from one session of firefox into another...why would you want to do this? Let's say you visit TKC in one session, then decide to do some research on a second session, but then visit a different part of TKC on the new session, with FF you can just drag the TKC tab from session two to add it to session 1 and keep it better organized.
Unfortunately, I've been having a problem with Firefox randomly crashing on me...I tried reinstalling but didn't solve the issue; so I'm stuck using IE at the moment.