It looks like Firefox 64-bit may have launched to the stable channels. I was poking around earlier when I saw 42.0 b1 x64 had landed. I checked for a stable build and when I added 64 after win, I got a successful download.
https://download.mozilla.org/?product=firefox-42.0-SSL&os=win64&lang=en-US
It sure looks like a stable build, is this official? I hope so!