what is APPBUTTON?

[joefiesta]

Topic "Appbutton overlaps tabs in latest firefox version #769" talks about something called "appbutton". I can not find out what this even is. Googling APPBUTTON gets me nowhere.
What is this "button"?

[Aris-t2]

The main menu button is also called application button or appbutton.

By default it looks like this:

This project optionally changes its appearance and moves it to titlebar like this was in old Firefox years ago:

[joefiesta]

can someone tell me where to find the .css to move the appbutton to the file menu? that looks really nice because the file menu bar has so much unused space. Thx!