We got 64-bit Firefox running on Mac OS X today.
I don’t want to post a build here because there are still some significant issues. We should be ironing those out quickly though. Not all of the patches required to produce 64-bit builds are checked in yet but we’re working on it.
We had to rewrite a lot of code to get this going. Thanks to Masayuki Nakano, Joel Reymont, Markus Stange and David Anderson for helping with some of the last bugs!