Mozilla engineers have decided to forcibly auto-upgrade some users of 32-bit Firefox to 64-bit versions. The only ones that will be auto-migrated are users who run 32-bits on computers with 64-bits. Firefox 56 is scheduled for release on September 26, 2017, while Firefox 55 is expected on August 8, 2017. 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 with more than 2GB of RAM.
Source: https://www.bleepingcomputer.com/news/software/32-bit-firefox-users-on-64-bit-pcs-will-be-auto-upgraded-to-64-bit-firefox/