Firefox is the App of the year! In my opinion.
An interesting note Firefox is basically Netscape with a few more bells and whistles. They both use the same source code.
FireFox's 'IE killer' qualties are its security and control features. I'm now paranoid to even open up IE to test one of my Web sites. It's a beautiful thing to run virus scan, Spybot, AdWare, et al and not get a single hit. Use IE for 10 minutes and look out!
IE is ultimately doomed by its original design and incorporation of MS active elements. If MS were to completely re-design IE to respond to FF, then it wouldn't even be IE any more - MS would have to eleminate every internal/back-end link.
Mozilla was installed on my computer the other day. What a great browswer.