I've used both for quite a while, and I'm voting for Firefox. IMHO, it has the feature that I need the most -- it renders pages correctly. Right or Wrong, Opera still has problems dispaying some pages out on the Internet. Most likely it's because the authors of those pages didn't code them properly, and IE renders anything no matter how badly written so they don't even know it's wrong.
Opera chokes on some of them, while Firefox can handle them more gracefully.