David Teller's Why Did Mozilla Remove XUL Add-ons? is the answer to this question, from someone who works on Firefox. Firefox XUL Add-ons are the old and more powerful form of addons, which have now been replaced by WebExtensions as of Firefox Quantum.
I knew a certain amount about this area (and it's an interest of mine, since Firefox WebExtensions still aren't quite as good for my addons ), but I didn't know all of the details and the article taught me things. I had never quite wondered what happened to Firefox's Electrolysis stuff, for example; the article answers the question.