Why Did Mozilla Remove XUL Add-ons?

During the past few days, I've been chatting with Firefox users, trying to separate fact from rumor regarding the consequences of the August 2020 Mozilla layoffs. One of the topics that came back a few times was the removal of XUL-based add-ons during the move to Firefox Quantum. I was very surprised to see that, years after it happened, some community members still felt hurt by this choice.

And then, as someone pointed out on reddit, I realized that we still haven't taken the time to explain in-depth why we had no choice but to remove XUL-based add-ons.

So, if you're ready for a dive into some of the internals of add-ons and Gecko, I'd like to take this opportunity to try and give you a bit more detail.

David Teller's been at Mozilla for nearly 10 years, so he knows what he's talking about. A good and detailed explanation of why Mozilla pretty much had no choice.