64-bit builds of Firefox were released by Mozilla in conjunction with 32-bit builds for nearly two years for the Windows platform. Mozilla about a month ago announced a change that stopped the creation and development of 64-bit Windows builds of Firefox in favor of concentrating resources on 32-bit builds and other projects.
A couple of reasons were given for that move, from missing or incomplete plugin support for 64-bit versions of Firefox to the builds being slower than 32-bit versions of the browser. A couple of reasons where internal in nature, the stability team for instance had problems distinguishing between 32-bit and 64-bit crash statistics.
What Mozilla may have failed to take into account back then was the backlash that it received after making the announcement. Mozilla reviewed that feedback and discovered that part of the 64-bit userbase of the browser did not use it for testing purposes but because they would run into the operating system's 4 Gigabyte memory limit otherwise. These users did not reach that memory limit because of memory leaks but because of very heavy usage of open tabs in the browser with some users having more than a thousand tabs open at the same time.
A compromise was found that may be in the interest of both Mozilla and the Firefox user base. Instead of retiring 64-bit builds of Firefox completely for the time being, Mozilla decided to continue creating those builds in the Nightly channel. Not everything will run along as smooth as it does now though as there are a couple of limitations and restrictions that current 64 bit users need to know about.
Mozilla will go ahead as planned and migrate all Firefox 64-bit users from the 64-bit Nightly channel to the 32-bit Nightly channel via automatic updates. Instead of stopping building 64-bit Nightly versions of Firefox, the company will continue creating those builds and users who want to continue using them need to download one of the future Nightly builds that are released after the migration to use 64-bit versions of the Firefox web browser.
Mozilla will make it clear to those users via the first run page that the browser is not supported and that the browser builds won't receive the same rigorous testing that 32-bit builds receive. The crash reporter will be disabled in 64-bit builds, and click to play enabled by default.
Firefox 64-bit builds on Windows are considered Tier 3 builds by Mozilla from that time on. A support page explains:
Tier-3 platforms have a maintainer or community which attempt to keep the platform working. These platforms may or may not work at any time, and often have little test coverage:
There you have it. Mozilla continues building 64-bit Firefox versions for the Windows operating system, and while the planned migration does not seem to be the best of ideas, it is likely something that 64-bit users can live with in the end. (via Sören)