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See “What are my options if I want to install unsigned extensions in Firefox?” in the FAQ here: https://wiki.mozilla.org/Add-ons/Extension_Signing#FAQ
If you’re on Linux, you may have that about:config flag even in your distro-provided Firefox Stable build…
If you type “about:profiles” into the URL-bar and create a fresh profile there, does it also happen in that?
(It will practically be like a factory-reset Firefox, except you can go back to your current data/profile.)