I rarely boot to Windows but I did now and on Nightly the default theme is taking a lot more space from the web than in other browsers.
Had to find browser.uidensity pref to get reasonable UI.
I haven't used compact mode on Windows before this.

(I guess on linux I have always used compact mode)

I think the problem only (mostly) affects maximized windows?

I'm asking because I spend most of my time with side by side windows (non maximized) and in that mode the difference is barely noticeable (a few pixels more than Chrome/Edge). In maximized mode though the tabs are definitely taking more space.

Yes, the situation is worse when the window is maximized. But that is only because other browsers use more space for the UI when they aren't maximized.
I would expect windows to be maximized normally, at least on laptops. And when one has a bit smaller laptop, the space lost to the UI is very noticeable.