Depends on a number of things - each of these browsers have good and bad points. I regularly use Firefox, Opera, Nescape (forerunner to Firefox) and now and then, Chrome, or MS IE (v6 on some machines, v7 on another, but it is absolutely my last choice and Safari doesn't get a look in, except on my really old iMac, banished upstairs for now).
Firefox - has lots of add-ons which can block ads and so on, and I use it primarily for checking websites (add-ons showing owner of domain, IP address where it is hosted, etc)
Opera - has some nice features such as bookmarks and notes which can be synchronised between multiple machines, (so notes added on a PC in the office can be seen at home and vice versa) and other 'odd' aspects like the order of displaying tabs is copied when you close them, rather than just showing the next left, or right.
Chrome - can pull a tab from one window to another (I have an extra screen plugged into this laptop, so have lots of windows/tabs open.
I use Firefox most, especially if I am comparing lots of items (you can save all the tabs used within a window in one go, and re-open them later, showing all the pages you had before). It's also handy for storing usernames and passwords (I have a couple of hundred) and I take screen dumps now and then, showing website / username / password and running to 10-15 screens, for my peace of mind)
As lazydog says, when a badly written website needs to be viewed, MS IE seems to "cope" while others are more strict. I remember seeing a website developed by someone local to me, where the navigation links would only display if one was using MS IE, no good for anyone with Firefox, as the HTML code was rubbish.
I use Netscape as well, because v7 and earlier have a built-in WYSIWYG webpage composer. I first bought a copy of Netscape Gold v3 in 1997 or 1998, so you might class me as a bit 'stuck in the past'. {I was sent a copy of Netscape v1.1 on floppy by a US ISP when I opened an account back in 1994/5 with them.)