The first thing that attracted me to Firefox, besides the security, was the tabbed browsing. That just makes it so much easier to keep multiple web sites open, plus you're only running one instance of your browser, so it conserves memory/CPU. But of course, IE now has it too. Ad Block is also fantastic, although it seems that more and more sites are incorporating ads that Ad Block can't handle. But besides Ad Block, there are literally hundreds of other add-ons available, much more than with IE, because as George said it's open-source, so everybody and their brother is writing software for it. Some of them are pretty lame, but there are add-ons to do just about anything you might ever want your broswer to do, if you're so inclined. Plus it's a cleaner, simpler interface, or at least it seems so to me. You don't have Microsoft adding all kinds of clutter to it that you have to turn off it you don't want to see it.