I have finally achieved my lifelong dream of getting Firefox down to only 1 bar along the top.
(i.imgur.com)
It's so beautiful I might cry.
I am using MrOtherGuy css - https://github.com/MrOtherGuy/firefox-csshacks
I use this one for ulr+tab combinated, with multiple options.
https://mrotherguy.github.io/firefox-csshacks/?file=multi-row_oneliner_combo_patch.css