Firefox 115.33.0esr released on Feb 24 is the last version to still support the EOL Windows 7, 8, 8.1. There will be no more updates for the old Firefox 115 ESR channel unless something warrants a minor update to 115.33.0esr for security or stability reasons. https://support.mozilla.org/en-US/kb/firefox-users-windows-7-8-and-81-moving-extended-support

Firefox 116.0 and later requires Windows 10, 11 to run.

A way to have a more current OS that is not Windows 10 or 11 is to perhaps dual boot with a recent Linux distro as you would then be able to use current versions of Firefox with updates from Mozilla.internal. However if your CPU is 32-bit then you will need to use Firefox 140 ESR (currently at 140.8.0esr) as 32-bit Linux support was dropped as of Fx 145.0 and later. Also there are not many 32-bit Linux distros now compared to 64-bit.

Turns out Mozilla decided very recently to extend the updates for the older Firefox 115 ESR yet again until later August 2026 to support the Windows 7, 8, 8.1 and macOS 10.12, 10.13, 10.14 for while longer.