> I really wish Firefox would make the UI more stable and make user chrome mods some kind of plugin system where I can pick and choose how to deeply customize the interface.

Firefox extensions before Mozilla decided to kill XUL had the capability to radically alter the UI. This is what made FF extensions so powerful.

There were arguably good reasons for them to remove XUL[1], but it created a void of functionality that previously existed. I still think that a more secure version of XUL could offer a better user experience, but unfortunately we're forced to settle on the severely limited WebExtensions standard nobody is happy with except browser developers.

[1]: https://yoric.github.io/post/why-did-mozilla-remove-xul-addo...