Firefox, despite its questionable UI design choices, is heavily customizable with the userChrome.css file. The author mentions having to figure out the correct ID/class names, but you can do it yourself just by inspecting the whole browser chrome as if it was part of an HTML DOM.
Browser Tool Box (open fly out menu -> More Tools, or Ctrl+Alt+Shift+I). This opens a remote debugging session, and you can freely select and customize any part of the chrome.
Browser Tool Box (open fly out menu -> More Tools, or Ctrl+Alt+Shift+I). This opens a remote debugging session, and you can freely select and customize any part of the chrome.