Mozilla engineers have decided that starting with Firefox 56 they will silently and forcibly auto-upgrade some users of 32-bit Firefox to 64-bit versions.
Obviously, the only ones that will be auto-migrated are users who run 32-bit versions of Firefox on computers with 64-bit architectures.
Mozilla didn't always provide a 64-bit version of Firefox. The first Firefox 64-bit version was delivered on December 15, 2015, with the release of Firefox 43. Since then, Mozilla has improved its 64-bit version, and last summer decided that it will make its 64-bit version the focal point of its development.
In spite of the obvious performance boost, adoption of the Firefox 64-bit version is still low. Mozilla says that 70% of Firefox users on 64-bit Windows are still running the 32-bit version.
Mozilla plans to move the bulk of these 70% to Firefox 64-bit. The only condition is that they have more than 2GB of RAM. According to Mozilla statistics, 92% of Windows users have more than 2GB of RAM, meaning most 32-bit Firefox users on 64-bit Windows will receive the 64-bit version in September.
Additionally, with the release of Firefox 55 Mozilla plans to modify the Windows stub installer to set up the 64-bit Firefox version by default for eligible users (Windows 64-bit and 2GB+ of RAM). Currently, the Windows stub installer will set up the 32-bit version by default, even on 64-bit platforms.
Firefox 56 is scheduled for release on September 26, 2017, while Firefox 55 is expected on August 8, 2017.
Google pulled off a similar move when it released Chrome 58 in May this year, moving 32-bit Chrome users on Windows 64-bit to 64-bit versions of the browser.