I've used Firefox since it's early days and I don't want UI changes. The move from XUL was bad, but I moved on. Since Photon, and even with Australis the UI changes have been undesirable to me.

The only modern Firefox feature I like is the omnibar and the "bangs" - * for bookmarks, % for tabs, ^ - for history, @ for named search engines etc. But we have lost far more features then we have gotten. We don't even have configurable search engines from the settings anymore.

Emojis always felt juvenile to me, call me a boomer (not really a boomer in age), but they are an eyesore for me.

My ideal Firefox build would regain classic chrome from pre-Australis days with integrated omnibar. The UI would never change. Along this I would like some plugins that were only possible with XUL. But I know we will never get this unless someone actually makes these classic forks as secure and performant as modern Firefox.

If it weren't for prefs and userChome.css I would have long since abandoned Firefox. Those are the only things that makes it still worth using.