Mozilla doesn't have a release version of Firefox 64-bit for Windows operating systems. Mozilla does have Nightly alpha-level 64-bit versions for Windows, but currently they are generated to only verify that Nightly channel changes aren't being broken on 64-bit versions of Windows.

There are a few 3rd party 64-builds available, like Waterfox, but those aren't official Mozilla releases.

https://support.mozilla.org/en-US/questions/972695?esab=a&s=+64+bit+version+of+Firefox&r=4&as=s

https://support.mozilla.org/en-US/search?esab=a&q=+64+bit+version+of+Firefox

There is nothing on the site to indicate it is by mozilla.org and Mozilla would never create such a site like this for Firefox builds, especially without any mozilla.org links around.

Also whomever created and is maintaining the site is apparently not aware that there has been official 64-bit Release builds of Firefox for Linux and for Mac OSX (universal binary) ever since Firefox 4.0. If I recall the owner was listing third-party builds before that were outdated.

Ok it looks like this unofficial Firefox 64-bit site is linking to another unofficial download site for these builds.

http://www.softsea.com/review/Firefox-64-bit.html
http://www.softsea.com/review/Firefox-64-bit-For-Linux.html

They look to be random Nightly builds from February 7 or 8 date time. I say random because the Nightly channel gets checkins pretty much everyday so therefore they get a update each of those days.

Mozilla has Linux 64-bit builds of Firefox 27.0.1 listed at http://www.mozilla.org/en-US/firefox/all/