So I was a long time Opera user (although I've always switched around with my browsers), however with the betas of FF4 I don't see myself switching to anything other than Firefox for quite a while.
Firefox 4 has everything I could want. The default interface looks very nice. It's got that OS X look, but is a little more mature than Safari since it doesn't have all that whimsical eye candy. That doesn't mean I don't think Firefox could use a little more animation. Safari, Chrome, and Opera are all ahead of Fox in this case. And the sync button in Firefox should actually rotate when syncing, not just light up blue.
I love the way Firefox handles bookmarking. I can't live without some of my extensions now. For example Greasemonkey, InstantFox for controlled searching right from the adressbar (better than how Chrome does it, but the same as Opera does it), QuickFox notes (Opera had this by default and I found a love for taking in browser notes), Tabscope (Opera had this by default), Thumbnail Zoom a cool way to mouse over pictures on many websites and see their full size version without clicking.
I guess there's not a lot keeping me on Firefox other than some bookmarks I don't have in Opera right now. Maybe if the next version of Opera has better extension support, or at least better extension developers I'll give it another go. But right now Firefox seems to be king. Hopefully in Firefox 5 they make it more multithreaded, refine the add-ons manager, integrate it better with OS X, and add more animations. That would make it the unarguable king of OS X browsers.