For one thing, there's no installer included yet, though that work is under way, too. For another, the software is still one of the very raw "nightly" builds for developers.
Support for 64-bit processors is one of the planned Firefox 4 features. Mozilla hopes to release Firefox 4 by the end of November.
The transition to 64-bit computing often offers a modest computing performance boost, but the main reason for the transition is getting around the 4GB memory limit of 32-bit computing. Since relatively few applications today require that much memory--or even whatever fraction remains after the operating system and other applications claim their share--the 64-bit change for desktop computing has taken years.