I believe the developer is using a simple built-in function in Firefox, but has made it readily available by clicking an icon.

Would be great for reading except it doesn't work when Column Reader app has been applied. [Edit: I just tried mFull addon instead and it works in reader mode].

Also, implementing the suggested bug fix might allow annoying websites to go full screen independently. Anyway, to exit full screen, just press [ESC] or [F11].