Firefox is updated too often to discuss every release but Firefox 78 is notable because it’s the last that will run on OS X 10.9, 10.10, or 10.11 (Mavericks, Yosemite, El Capitan). If you’re on one of those, you should probably use Firefox ESR (Extended Support Release) so you’ll still benefit from security updates for a year. Firefox 79 and newer will require an OS named “macOS.”
Chrome is still supported on OS X 10.10 Yosemite and newer but it seems to drop support for older Mac OS’s every two years and 10.9 was dropped two years ago.
I wonder if there will be any community support for older OS releases, much like how Classilla supported it on MacOS 8 and 9 until 2014 and how TenFourFox appears to still be providing support for PowerPC platforms.
It probably won’t be possible for community builds to support all the latest features, but if they can take the source from 78 and back-port security fixes, that may be enough for those who are unable to upgrade their Macs to Sierra or later.