There are a coupe of *unofficial* 64bit versions of Firefox, and there are many 'Architecture (read CPU) optimized builds out there as well.
A good place to start reading would be Third Party/Unofficial Builds • mozillaZine Forums
As for direct links to Firefox x64 builds, try: Mozilla for Windows x64 platform 64bit - x64 - Firefox - x64 version
and a post from right here in the forums (a small hint that you should search first): Firefox X64!!
There is no Flash player for 64bit broswers yet
This causes some issues, obviosuly - hence the lack of real development of 64bit browsers.