If you are looking for speed then safari is said to be better, but firefox is supposed to be able to handle more complex websites better, it also has some cool features that safari doesn't. But for all you google chrome geeks, the word is that a mac version should be out soon! And my guess is this will be the fastest!
Personally i use firefox as my default. It's stable and for me is more user friendly. my personal favourite is the seo rank check add-on! Check it out it is a lifesaver! http://tools.seobook.com/firefox/rank-checker/ .