While Firefox add-ons like Tab Mix Plus can move tabs to bottom and Tabs on Bottom puts tabs below the navigation bar, Bottom UI is designed to move both the navigation bar and tabs to the very bottom of Firefox window. It felt a bit strange at first but it didn’t take long for me to like the new look. Now I can totally focus my attention to the contents of web pages without the crowded toolbars that sometimes give me headache.
If you don’t like the top being empty, the extension has option to make the menu bar always visible, which also shows Firefox control buttons (min, max, close) on the right top side automatically.
It is possible to keep the control buttons visible even though the menu bar is disabled or hidden. However, the location of the buttons can only be set either on the navigation or tab bar at the bottom.
Bottom UI is conveniently restartless. Get it from here, install it and enjoy the clean view of the top.