What has seemed like an eternity of flip-flopping about having a Windows 64-Bit (Win64) version of Firefox may very soon come to an end. Earlier this week Bug 1180792 was filed to enable 64-bit windows builds on release channel. In October 2014, Mozilla had started talking about having a Win64 release as early as Firefox 37 for March 2015. Firefox 37 came and went and still no Win64 released. There have been Win64 versions for Firefox 38 and 39, but those never offered outside the Beta channel.
So, when will be seeing this Win64 release? There is still some debate about that right now. Ideally, it should be Firefox 40 which is scheduled for August 11th. On the other hand, Windows 10 is due for release at the end of this month and most of all the other browsers will already have a 64-Bit Windows version. In that case, it could be pushed out as Firefox 39.0.1. However, the point of ‘point’ release is to address bugs, security issues, regressions, etc. NOT to introduce a new feature (if you can call Win64 support a feature). This will be discussed in the channel meeting scheduled for later today.
One thing to keep in mind is when the Win64 release is pushed out, it is not going to be on the main Firefox release page. It will be on the fully localized versions page which can be accessed via the Systems & Languages link on the main page. Eventually there will be mechanism in place to automatically detect if the user is running a 64-Bit Version of Windows and offer the Win64 release.
Update (07/08/15 1045): Bug 1181014 was filed on July 6, 2105 which indicates a target release for Win64 builds on the release channel with Firefox 40 on August 11th, 2015.