Firefox is good, but IE7 has adapted a lot of the Firefox functionality that originally lured me over in the first place. Most notably tabbed browsing. The built-in RSS reader in IE7 is nice too.

The problem I had with Firefox is that a lot of sites, particularly older ones, don't display properly. I got sick of things look all out of whack on sites or not working right because people are too dumb to make their stuff cross-browser compatible so I went back to IE. Granted not everything works right in IE7 either, but it's not as bad as Firefox.