A lot of people complaining about the UI changes on other forums. As my daily browser is Firefox Nightly (since long before Nightly was a thing, and long before Minefield) I saw these introduced incrementally, which lessened the ‘jarring’ effect.
And - apart from the new ‘update’ UI, which of course I see all the time - I hadn’t really noticed the other UI changes until I read about them in the 89 release notes. And I do like the tab focus.
Is this enough to save Firefox? Probably not.
Will I ever use a Chromium based browser? No.