I'd hate to break it to ya, but Chromium browsers do this shit too. I have an older Intel Mac that I downgraded to Mojave a few months back, and
no Chromium build would work on it: not Edge, not Brave, not even Ungoogled Chromium even if I tried to compile it via Homebrew. Not only that, but I had to upgrade to Catalina for v128 ESR to install, and even then, the clock's ticking on Catalina support too. The modern internet does
not like old operating systems, and they
will be left in the dust, unfortunately. Apple is far worse in this regard than other companies, but it is what it is.
As to your Firefox dilemma, it's specifically related to the
root certificate that comes in older builds prior to v115.13 (ESR). Rebuilds of Firefox like Librewolf and Waterfox are hardly distinct from upstream, so they'll most likely be affected too. I presume you're still holding out on migrating from Windows 7/8.1, since that's what this root certificate change will impact the most. I'd suggest migrating to Linux or something, but Linux distros just don't hit the same as classic Windows 7.