I hope Firefox does thrive. It seems to be the best browser for web developers. I use several plugins to assist in debugging websites (Firebug and Firesizer for example), and the ability to view image info is also handy - Chrome, by default, does not make that easy.
It's possible that similar functionality is available for Chrome, but it's also nice to have one lean browser for real browsing, and a plug-in laden one for web development. IE I only use when I want to see what it breaks, although to be fair IE9 now does a much better job at rendering things the same way as Firefox and Chrome.
Anyway, I still find Firefox useful and hope it has a future. At this point in time, I can't see the lack of a 64bit version being a major drawback.