Your point that extensions to modify the UI vs. userChrome.css is well taken, but I would argue that it's apples and oranges. I did a poor job of explaining myself. I'll attempt to elucidate and apologize if I do a poor job again. My argument regarding breaking extensions was in the context of allowing users to modify the browser's UI/UX to their liking.

Installing an extension is a common practice performed by many people who possess little-to-no "computer skills." (for lack of a better way to put it.) Just about anyone can search for an extension, perform a few clicks, and install it. And this used to be a popular method for those who disliked UI changes Mozilla foisted upon them so that they could easily return to a UI they enjoyed. Unfortunately, the UI modifications that can now be made with extensions are minimal. Users are encouraged to modify userChrome.css instead. While this may seem trivial and perhaps preferred by some in the HN demographic, this effectively takes it out of the mainstream. Most users are not going to find their FF profile folder, create a chrome folder (which doesn't exist by default), and start coding css to make their browser look different. Nor are they going to have any idea that there are projects on github which would provide solutions for them. But let's say, for argument's sake that one did go to the trouble. Would you like to move the tabs below the address bar? Good luck with that. And then watch your css no longer work when Mozilla makes further changes making your modifications ineffective. There is one project that does manage to move the tabs below the address bar but it leaves a blank area above the address bar where the tabs would be. Mozilla has dug in on their tab bar dogma, user preference be damned. We're seeing it again with proton. It seems to be universally reviled, but guess what: you're gettin' it whether you like it or not. Want to use your operating system's printer dialog box instead of FireFox's incomplete and buggy one? Too bad.
https://support.mozilla.org/en-US/questions/1322589?page=2#a...

I also understand that people create derivatives of open source projects as an intellectual pursuit, which is wonderful. However, I used Waterfox as an example. Indeed, Waterfox appears to have begun its existence in much the way you describe. But fast-forward to now, on their website's main page, they trumpet their tab bar thus: "Fan-tab-u-lous - Everyone likes to use their browser in a specific way." And indeed, in the preferences there is a simple way to set the position of the tab bar. Why would that be so hard for Mozilla to do in Firefox? Why? They also highlight, "Limited Data Collection," "No Telemetry," and "The most extension support of any browser." Do you not think it safe to infer that this may be their current raison d'être and how they would like to differentiate themselves from Firefox?

Yes, I know there are alternative projects. My criticism is directed primarily toward the Firefox UI team. It is a mystery to me why an organization that expends so much energy and money marketing itself in an attempt to get new users, regularly angering its existing users to the point where they want to bail. This article is nice explainer: https://www.inc.com/karl-and-bill/its-cheaper-to-keep-em.htm...