I agree that the Firefox user interface feels way too cluttered these days. I know this is a highly unpopular opinion, but I find the way that the Suite (SeaMonkey) allows you to collapse/expand title bars infinitely more functional than whatever Mozilla has been doing over the past decade.
https://i.imgur.com/b5ZpLaO.jpg
https://i.imgur.com/Se4rYJc.jpg
https://i.imgur.com/ynphRKv.jpg