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which have now been replaced by WebExtensions as of Firefox Quantum.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.
itself via @mhoye.)