[_Microft]

Firefox (currently) has a setting called browser.compactmode.show which makes the UI more compact when set to true. You might want to check it out?

Using this setting, tab bar plus navigation bar take up 72 pixels vertically for me.

[dmitrygr]

They are on record that it will be removed "real soon now". Apparently "nobody uses it"

[_Microft]

I did some searching and they seem to keep the about:config option:

https://bugzilla.mozilla.org/show_bug.cgi?id=1703254