For Linux and MacOS it's been available for a long time.

I've also used the 64-bit Firefox on my Windows for some time now, but it's not the default download that they offer. See it here: https://www.mozilla.org/en-US/firefox/all/

In fact, 64-bit Mac will be the only version supported as of Firefox 53. This will halve the Mac installer size. 32-bit and Universal Mac builds were dropped in Firefox bug 1295375: https://bugzil.la/1295375

"Firefox for Other Platforms & Languages" in footer of that page.

That link used to be more prominent in the past, right under the main download button.

The fork is called Waterfox, and yeah, the 64-bit Windows installer is relatively new.