When you say 64-bit Firefox you mean for Windows as Mac OSX (combined 32/64) and Linux has had 64-bit for Release ever since Fx 4.0. For Mac and Linux it was both easier and needed.

For Win64 the earliest there may be is Firefox 37.0 in a first phase if all goes well in best case scenario. https://wiki.mozilla.org/Firefox/win64

Since Win32 works fine and Win64 is not needed in order to run Firefox on 64-bit Windows it has not been a priority to get through the different hurdles in having Win64 Firefox for Release until again more recently.

That is something that has not had much work on it until recently.

IIRC work is being stepped up but I am not sure any Release is imminent. I found this

• https://groups.google.com/forum/#!topic/mozilla.dev.platform/7tEPeFU4TMI

This suggests it could be as early as Firefox 37

• https://wiki.mozilla.org/Firefox/win64#Roll_out_Phases

If you ask briefly and politely on one of the fora | mailinglists | groups

• https://www.mozilla.org/en-US/about/forums/#dev-platform

You may get an answer or at least be told where the correct place is to ask