As simple as an extension comes, but it works. With FF57 hide caption titlebar plus probably wont be available anymore. On most Linux window managers though you can specify that certain applications are not to have window decorations; set your window manager to hide decorations then install this and move it to the tabbar. Do the same for close and minimize extensions and you have a working solution.

However, the one flaw is that there is no choice of color (why I took away 1 star). Its hard to see the buttons when you have a dark theme as the icons are dark themselves (works fine on a light theme). So suggested improvement is to add a light/dark theme option where the icon colors are changeable.

Would be 5 stars if ...
a) choice of colors ...
b) you gave some explanation on how to remove the minimize/maximize/close button on FF menubar with Stylus for example (or made it possible via your addon ...)
Thanks anyway ...