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I use 3 different ones .___. Apple Safari is my main; Firefox is used by my parents; which is my back-up. But lately it's been silly so I downloaded Google Chrome just incase.
I use firefox, mainly because of the Adblock+. It's simply amazing. Plus I've been using it for a long time now, and it's hard to adapt to a new browser so quickly.
For me, it's Firefox in Windows 7, mainly because I don't trust the security (or the lack thereof) in IE. You know, gotta make sure my head is on straight with all of this annoying adware and junk.
As for Mac OS X, it's a toss-up between Safari and Firefox.
I use Firefox, mostly because I have quite a few addons that I use with it that aren't compatible with other browsers. I use Chrome as a secondary browser, but I just can't get used to anything other than Firefox to use on a daily basis.