Earlier quoted context omitted.
You have to use Firefox Developer Edition and change 'xpinstall.signatures.required' to True in about:config
What about the part where I wrote > Yes, I want Firefox stable. I don't want to run a buggy browser. ? Is the developer edition running the same code as stable?
https://hacks.mozilla.org/2017/04/simplifying-firefox-releas... (edit for better link)