Hands down Firefox. I tried IE7 for about a month and when I went to uninstall it, it was corrupt so I had to reinstall windows. Says alot about the company as a whole.
Maybe it's just me but I have a really hard time finding the reload button on the new IE. It's like they went out of their way to make it really hard to find.
firefox all the way!! it does use a lot of ram for me (45-50 tabs open) but i will never ever use IE7
Firefox uses much less resources, and its faster for me. In IE, each tab you open eats up like 2% ram. In Firefox, i open 10 tabs, it finally eats up 3% ram.