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 impossible to implement.
This "Have It Your Way" capability would be a profound differentiator and user value proposition to stand out from the sameness of Chrome and Safari. I still use FF but to make it usable I have to install my own UserChrome.css and User.js which isn't easy for non-tech people. On top of that I regularly have to go "fix" new UI behaviors that Mozilla's designers keep shoveling into the UX in a constant game of Whack-a-Mole.
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 was losing tons of users to Chrome long before that changed. And Firefox was significantly slower and taking longer to integrate new features due to that customization around the old extensions model.
This "Have It Your Way" capability would be a profound differentiator and user value proposition to stand out from the sameness of Chrome and Safari. I still use FF but to make it usable I have to install my own UserChrome.css and User.js which isn't easy for non-tech people. On top of that I regularly have to go "fix" new UI behaviors that Mozilla's designers keep shoveling into the UX in a constant game of Whack-a-Mole.