Firefox

Firefox has 50 in-house developers, but, in open-source tradition, 1,000 volunteers worldwide regularly contribute code. The result is what its users will tell you is a superior piece of engineering to Microsoft IE. Firefox was first to popularize tabbed browsing and first to integrate pop-up blocking, spell checking and antiphishing into the browser. It has a nifty word finder that doesn't require opening up a separate dialog box as in IE. Some 3,000 software extensions have been written to make it more versatile... Firefox had only a two-day window of exposure, on average, to malicious hacker exploits, the lowest of any browser. ~Quentin Hardy

This article explains the woes that mozilla has faced in their "david and goliath" fight against microsoft. I think firefox is better in general, but obviously I'm biased against closed source development. I truly believe that an open source project will always be better than a closed project. There are just simply too many people to debug the program to mess it up too much. The best thing about firefox is its security- obviously. There are people working constantly to patch up vulnerabilities in the firefox code. Not to mention the cute little fox in the icon is like so totally better than that stupid "e". SO THERE!