This extension allows users to add blank space above, and to the right of, their tab bar. It can also be used to remove the extra padding, added by Firefox, in Windowed mode.
Note: In Mac OSX, there might be a little bit more padding that I could remove. I am still looking in to it.
It is included in "LighterWeight Themes for Australis", an extension which modifies the look and feel of Lightweight Themes (formerly, "Personas"), which is available at: https://addons.mozilla.org/en-US/firefox/addon/lighterweight-themes-for-austr/