Mozilla this week announced it would automatically move users running outdated versions of macOS to the Firefox Extended Support Release (ESR), an edition that provides security updates only.
The move, a first step towards dropping all support, will take place June 30, when Mozilla releases Firefox 78.
On that date, users of Firefox still running OS X 10.9 (Mavericks), 10.10 (Yosemite) and 10.11 (El Capitan) on their Macs will instead be shunted to the extended channel and given 78.0 ESR. While that and Firefox 78 will be identical, when the latter shifts to version 79 four weeks later, ERS will remain at 78, increased to 78.1 to mark its first security update.
Firefox ESR and its limited feature changes were designed for enterprises that valued stability over sexy new functionality. Mozilla has used it before to wind down support for aged operating systems; three years ago, it pushed users who relied on Windows XP or Windows Vista onto Firefox 52 ESR.
For the next year, Mozilla will deliver security updates to Firefox 78 ERS running on Mavericks, Yosemite and El Capitan. In July 2021, those patches will stop and anyone stuck on one of those versions of OS X will be taking risks if they're browsing with Firefox.
Apple abandoned those flavours of OS X some time ago. The last security update for El Capitan, the youngest of the three, was in July 2018.
By tradition, Apple supports only the three latest versions with security updates. Currently, the trio in support are Catalina (10.15), Mojave (10.14) and High Sierra (10.13) from 2019, 2018 and 2017, respectively.
Other browsers continue to work on some older Apple operating systems. Although Google shut down Chrome running on Mavericks in 2018, the browser remains supported on Macs running Yosemite and El Capitan.
More information about Mozilla moving some Mac users to the ESR build can be found online.