Restore "User Customizable" as a top-level priority. I came to FF initially over 15 years ago because add-ons could change almost anything including fundamental appearance and workflow. When changing to the new, far more limited add-on infrastructure ~5 yrs ago, Mozilla promised that new APIs would be added to re-enable hugely popular add-ons like Tab Mix Plus yet this and other add-ons users relied on still remain i…
Firefox is already more customizable than Safari or Chrome. If Safari and Chrome were growing in userbase due to being more customizable than Firefox, it might make sense.
It would be really nice if they went all-in on those custom features, but I suspect it might need to come from the Servo base engine, first.