The required signing of addons (even local installed ones) is the main reason i won't use firefox anymore. And no, using a developer build (which might work or not) isn't fixing it.
Even Edge allows me to run unsigned addons (even if it's with a warning, but for my use case I am fine with that one).
If you can't do that maybe you shouldn't install custom extensions.
(Joking, but maybe mozilla thinks that way?)
Anyway if you want to be able to run normal Firefox version (no beta,developer or esr) and want to install custom extensions there is an easier way but not well know.
Actually I had set a policy to block automatic updates. Sometimes they break things, and having a profile backup before updating to roll back in case of breakage is a must. So it is a win win for me.
Even Edge allows me to run unsigned addons (even if it's with a warning, but for my use case I am fine with that one).