Fancy saving a bit of precious real estate on that monitor or screen of yours?
This is Windows only so I can only guess at what it does from the pics. It looks like it puts all the top menu bar menu items like File, Edit, View etc into a smaller drop down menu saving(?) space.
https://addons.mozilla.org/en-US/firefox/addon/1455 Extensions/TinyMenu - Arantius - Trac