I've been using Firefox for over a decade now, and although I do agree with some posts here about issues with the new UI. I believe we are going about it the wrong way.
Let me preface by comparing to another recent example that I have encountered. With Apple and their new CSAM scanning aka backdoor no matter how much we kick or scream they won't back down because they are a gigantic corporation that is also beholden to the government. In Firefoxs case, yes they are a business, a not for profit one at that. However, whatever Mozilla does to Firefox can be worked around by some tinkering or getting an alt. For example I use IceCat now because I don't like the telemetry and some bloat in FF like Sync and Pocket.
HOWEVER, if Firefox would actually stop development like so many people are crying/wishing would now happen just due to some inconvenience, we would be down to what, 2 main browsers and their disgusting spinoffs. There would be Safari with Webkit and all the Chromium/Blink aka malware browsers like Chrome, Brave, etc. Do you really want to live in a digital world like that? Right now, although it has shrunk somewhat, the Firefox voice is still out there. It can rebound and when the new Internet standards rise the FOSS community will still have a champion in the Internet realm.
If Firefox would actually stop development, there would just be handfuls of developers here and there making their own Firefox 2.0 (like my IceCat) but it would be nowhere near as secure as now due to the amount of people working on it, and the state would stagnate until eventually you would have to pick Safari or one of the malware/ Chromium/Blink browsers.
You don't actually expect another group to rise out of this and create a new engine all on their own right? That would be an insanely massive undertaking. This isn't 1990s anymore and the Internet is a terribly duopolized place with Apple and Alphabet/Google. Hell, even Microsoft killed their own shit and went with Chromium to make Edge.
You think another FOSS browser like FF would ever rise if MICROSOFT couldn't compete? Good luck with that.
Our only chance at FF going forward is open communication, perhaps on boards with CIVIL discussion. Show your points and reasonings without being a dick. Draw up some templates of what you like and submit it to the FF dev team. Going around berating them will not make them listen to you any more, I can tell you that.
Do you yell and scream like a little bitch at a stores customer service or restaurant when something is wrong? No, you act like a normal person and explain the issue. If at that point Mozilla still won't listen to us, then fuck them and they can kill Firefox. I will use FF/IceCat as long as I can while feeling safe and then make the unfortunate choice to pick one of the malware browsers.
For now have faith in the Mozilla team that has treated us right (maybe not always correctly) for the past NEARLY TWO DECADES FOR FREE and give them some time to react. Some of the issues I hear are already fixed in the upcoming nighty 93 build. Why not install that and submit more feedback?
May Kismet be with you