This theme is based on Pseudonosaur's userChrome.css and designed to minimize the size of the navbar to get more out of Firefox without compromising functionality.
Navbar hides itself when not in use. Simply hover near the navbar to expand. CompactFirefox gives you more screen real estate by shrinking down the navbar.
Auto-hide enables the user to get the simplest and cleanest look. It is almost three times smaller than Google Chrome's navbar!
Download any theme from Firefox Add-ons to customise Firefox 
Add or remove tools from the navbar easily
Interested? Installation instructions here