Firefox 4b7 has been released for quite some time already, but so far you can't find the 64-bit Mac OS X version in any obvious places. And no.. the usual Mac OS X version you get from Mozilla is a 32-bit binary.
But there's a way to get it...
First, you need to download an older beta, which is publicly availble in 64-bit binary:
Then, you update it. Help -> Check for Updates...
It will magically update itself to 4.0b7, 64-bit. The update logic in Firefox goes to a weird server called aus2.mozilla.org which doesn't give you any trivial public file listing - so you have to rely on the update process to help you there.
Edit: From FF4b8 and up, the Mac .dmg is already a 64-bit binary by default.