Total wrong forum but meh, whatever. And are you seriously even asking that question haha? Firefox of course.
I think it's not right place for such topic...by the way, I'm using both browsers. Firefox is on the 1st place, Opera the 2nd and IE on the 3rd place by the frequency I'm using.
Previously, I used IE a lot... but now I'm addicted to firefox.. And as a webmaster.. I hate IE a lot .. it has lots of bugs..
Since we are into web development and technical troubleshooting we have to install all major browsers in our PCs. So we have IE, FF, Chrome,Safari, Opera installed... But if given a choice or if we have an option we will uninstall all of them without any hitch and will only use FF. Reasons : Fast, ease of use, Book marks toolbar, Thousands of Addons, less buggy ( compare to IE ), was first to introduce tabs ( and still have the best and fastest tabs to use )...
Would have been nice if you have indicated why you wanna know this. On average internet users have equal share between IE and Firefox. To me IE is a total crap, firefox is great as it confirms all standards.
I am currently using Firefox.. I can use tons of useful Addons on Firefox compared to Internet Explorer..