I mainly use Opera, but it lacks a good RSS reader, so I also use Firefox from time to time.
I think Opera is the best, but Firefox is nearly just as good and resembles MSIE more, so it's understandable that so many people use Firefox.
I've also found that with the combination of Opera and Firefox I almost never need MSIE, so that is something I recommend people try before they give up on some pages and goes back to MSIE.
For all those who still use MSIE: It is buggy, it has a number of security holes, it does not have a full implementation of CSS1 and nothing of CSS2, and it lacks a lot of features that Opera and Firefox has! Both alternatives are free, so there is no reason not to change.