Firefox, on both Windows and Mac. It's the best one for me.
But I am only 2 steps away from switching to Opera. I keep checking in once in a while and by now it's faster and smoother than Firefox. With the extensions I can have almost all the functionality I have with Firefox, too.
But the adblocker is still inferior to the one(s) for Firefox. It doesn't free up space of blocked elements for instance. Also, I HATE the white flash Opera gives you every time you load a page. Instead of just switching to the next page's layout, Opera always starts with a white page. It's only visible for a split second but I feel like I'm about to get an epileptic shock from using it at night/on dark websites. You can never get rid of it completely. Until they finally get this fixed --though I'm not sure if they are aware of the problem, talk about "It's not a bug, it's a feature." etc-- I'll be using Firefox.
Not a fan of Chrome at all, neither of IE, new or old.
I loved Firefox for a time. I became quite attached to the add-ons. But, alas, the notorious memory issues weighed me down. When a browser makes you want to physically punch it, and it's not a markup discrepancy in IE, something has to give.
I have most all browsers installed for testing, but I've managed to settle into Chrome. I used Safari once. Didn't last. And I used Opera as my first anti-IE browser in my early college days. Can't really say why I strayed, but I did.
Still in love with Firefox and for me it's still the best browser. Eventhought recently I have had problems with Firefox to resolve all the feature from DeviantArt and somewhy it still gives a bad xtml... But anyway.. For that bit I'll use Chrome which I certainly still dislike. It's not so handy for me.
It's def leading by a good margin. I officially switched to it full-time just in the last few weeks, and I'm still enjoying the speed like it's some new thing. I had to wean myself off of my FF plug-ins, but I was fed up with the memory leaks and bloat.