Shipping Win64 builds by default to a wider group of users is a high priority in 2017 (stability being a big driver of it), but there's still some bugs blocking it. The work is being tracked at https://bugzilla.mozilla.org/show_bug.cgi?id=558448 and various dependencies.

Mozilla's wiki has a schedule for increasing Win64 Firefox usage in 2017:

https://wiki.mozilla.org/Firefox/Win64#Schedule

FYI, this default has been flipped on Nightly, and that change should make it in to the version 53 release in April (barring any huge problems found in the 64-bit build).

I didn't even know there is a 64-bit Firefox installer. I remember there being a fork that would build it for x64.

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.

Why is this? Are most windows users using 32-bit windows? (I refer to windows, because most other OSs will use their own package managers, probably).