Because every other release is a memory black hole that makes my laptop lift off from the madly spinning ventilators 60 seconds after launching the browser. It’s been like that for a decade or more. That is why I’m not using FF, and on Opera as default browser (as well as TOR).

It's #Foxtober. Why aren't you using Firefox? Seriously, why not? It's not run by a giant corporation. It's got a cool community. It promotes browser diversity.
i don’t have this experience obviously 🙂
i run firefox, the release version, currently firefox 69.0.1, on 3 windows machines and 2 macs without any issues. Problems like yours often because of add-ons.and usually starting a new profile [2] and/or a firefox refresh [1] fix the problem.
good luck!
…Roland
[1] https://support.mozilla.org/kb/refresh-firefox-reset-add-ons-and-settings
[2] https://support.mozilla.org/kb/profile-manager-create-remove-switch-firefox-profiles