In my UI (on desktop), tabs take up the title bar and all the other necessary buttons fit in the row with the address field. I've also got a bookmarks bar below that but it's optional. There's also the optional sidebar that I do use but mostly keep hidden.

For menus, there's the tab menu that is a button on the tab row and is mostly filled with open tabs plus 4 other items. Then there's the main menu with 19 items. Tbf, one is undesired (log in for sync) and at least 7 are redundant (as in if they weren't there, I'd still have easy access to their functionality), but I find menus are easier to discover features through (and hate how MS wants to get rid of them) and am ok with the redundancy.

There's also various context menus, but I've never found them to be obtrusive.

This is the state after some customization, but not a crazy amount. If I install FF on a new system, I can usually get it to a point where I'm happy with it pretty quickly.

If you mean the mobile version, I can understand that a bit more but personally prefer the screen space to be used up to expose more functionality. On mobile, I've felt like FF gives me the most power that feels closest to what I can do on a desktop, though tbf it's been a while since I used other browsers (excluding electron stuff or apps that integrate Chrome or Google Web view for web browsing rather than letting me use my preferred browser, though I've never felt happy with the more minimalist UI instead of the capabilities FF exposes).

I also haven't tried librewolf and don't know if the same UI is possible there.