Pretty much all the addons I have have been disabled and can’t find a way to re-enable them because they aren’t signed.
I have some addons that I have hacked together myself and aren’t distributed anywhere, can I install them even if they aren’t signed?
Please have a look at the announcement:
For your own unpublished Add-Ons you can flip the pref xpinstall.signatures.required in about:config (but this might only work in beta and nightly releases).
Changing xpinstall.signatures.required to false also works in FF 66.03 (It saved me a LOT of trouble). Once Mozilla fix their problem properly, remember to change the setting back to true.