I use the most recent of Firefox because it has tabs, it looks nice, it has a few plugins I like, and it works for me. :D
Oh, and now it has spellcheck; I need that, I really do. XD;
IE used to be slow on the old computer and it didn't have tabs (I didn't bother to upgrade to IE7 on it), so I didn't use it then. I don't know about this one, I haven't really tried it. I might have to someday, just to see how it works. I used Netscape for awhile, but I liked Firefox better once I got it. I've also tried Opera for a little bit, but I didn't see much of a difference between it and Firefox. Well, for the features I use in a browser, anyway; I know Opera does have things that Firefox doesn't, but I never used them.
I remember the days where I wanted IE and I thought Firefox was the evil... that changed fast once I actually tried it. XD