More secure. I'm sure IE has come far, but even on IE8 systems I still see tons of spyware installed.
Add-ons. If Firefox doesn't have a feature I want, I can add it easily. Flashblock for instance, or a built in music player. Firefox has basically become a platform onto itself, and I'd rather have it all integrated than run multiple apps. (if you really want a good music player plus web browser, get Songbird, it's built on firefox)
Speed. Firefox is so much faster on Javascript heavy webpages it's not even funny. And that's with the official release, the beta 3.5 release is way faster than that even.
Besides, Firefox is open source software and basically the reason any concern over "standards compliance" exists. I'd much rather use one of the top end web browsers that gives me an identical experience on any platform I choose to use, rather than proprietary software that isn't updated with the new features and ideas the web brings about. (opera however, while proprietary, is worlds better than IE in staying on top of worthwhile features, and it's still faster than IE, I keep opera on my thumbdrive so I don't have to use IE at public computers)
This. Adblock is all that keeps me on Firefox. Chrome is so much faster it's ridiculous.
Try the Firefox 3.5 beta. It seems identical to chrome in speed.