There is no official 64 bit version of Firefox for Windows released yet. Mozilla are in the process of developing one but it will not be available until after Firefox 4. The download it is offering is not official.

Dear Firefox,

                     Please release a 64 bit version fast.The computer is evolving very fast and windows 7 users will not be able to see the aswomeness of this browser.please make it fast.
                                                                                                                     Sincerly,
                                                                                                                     Geian Lingan

The 32-bit builds works just fine at moment anyways. At the moment the 32-bit builds are faster compared to 64-bit build built on same source.

As for officialy supported 64-bit Firefox release for Windows, well the earliest that may be is currently Firefox 12.0

check for the firefox nightly releases, there are 64 bit releases that far exceed the speeds I've ever seen from a 32 bit browser, and they allow for almost all add ons that I use as well.

boo1019, they are not officially supported builds though as they are mainly for testers and devs and for breakage testing as it is possible to break 32-bit without breaking 64-bit and other way. If a server for such build goes down or a regression caused by a patch happens then it does not have the priority to get working right away like the regular builds do.

You also need 64-bit plugins for those builds.

https://developer.mozilla.org/en/Supported_build_configurations