As for myself, I am currently running Opera; because it's free, and because the OS X port of Firefox is slow and buggy. Opera runs way faster, is way less bloated, and also surfs faster than Firefox.

I've been a Firefox user since the late Firebird era, and Opera never gave me the same feeling while using it, so I've been holding onto the good old bird because of it's unique feel. In OS X, Firefox doesn't feel the way it did anymore. That's why I will give Opera another try, it's a decent browser after all.

SAFARI IS NOT AN OPTION.