i should have been more specific: client-side decorations.

its the separate titlebar. by right clicking an empty spot in the tab bar and clicking “customize toolbar”, there will be an option to disable it, and get a few more vertical pixels of real estate. it will work more like windows and gnome-based environments.

if you want even more vertical space, go to about:config, set compactmode to true, then a more compact ui will be available in the customize toolbar menu.

also obligatory ublock origin as your adblocker might do better in hiding this sort of whitespace.