Firefox 116.0 and later including the current Firefox 132.0.1 and Firefox 128.4.0esr requires macOS 10.15 (ten.fifteen) or later to run.

Firefox 115 ESR requires macOS 10.12, 10.13, 10.14 to run https://support.mozilla.org/en-US/kb/firefox-users-macos-1012-1013-1014-moving-to-extended-support

The good news is that Mozilla has extended updates support for the eol Windows 7, 8, 8.1 and macOS 10.12, 10.13, 10.14 til Firefox 115.21.0esr in March 2025 unless Mozilla extends again. Pretty much every other major browser dropped support back in Jan/Feb of 2023.

Ok so let me get this straight: my version of Mac (mid 2010 therefore a dinosaur in computer terms) OS version 10.13.6 and Firefox is version 115.17.0esr. (I don't think ff lets me upgrade any further.) Does this mean after march 2025 I'll be vulnerable to whatever lurks in the interwebs dark places? Is there no way you clever geek/nerds can tweak the sytem to make it last longer? (sorry, I really have no idea how this thing works nor what most of the terms mean: i just want to be told relax, we have a new workaround, you can carry on using your computer till the cows come home..) It just galls me that while its working fine, I may have to throw the thing out if it becomes a security issue, if there are no available alternative browsers out there. Any ideas anyone?

Mozilla will be making the decision in early 2025 on whether to extend with more updates for Firefox 115 ESR. Whatever last update of Firefox 115 ESR will still work however yes you may potentially get vulnerable to critical exploits that does not get fixed in the future if you are not careful.

I do not know if there are any third-party builders or projects that will back port security fixes as such effort was done for a long while for PowerPC Macs for over a decade with the TenFourFox project for example.

A option may be to dual boot with a Linux distro but depending on your CPU and other things it may be more work compared to relative ease of installing Linux on normal computer systems.