Yup.. i seen the download by accident early today for version 42 while downgrading.
I thought i was hallucinating LOL
Google search for the Firefox FTP site to get v42. (or any build/version)
OR..
The main page has fresh update 43 with x64 available.
Just now i seen a news post about it at Neowin.net
http://www.neowin.net/forum/topic/1282048-mozilla-launches-64-bit-firefox-for-windows-update/Note:
Plugin issues..
I never tried it either.
so..