This is exactly what is needed if you have gone for a title-bar-less Firefox setup like I have. I use Gnome in Linux with gnome extension that moves all app titlebars to panel at top of the screen, hiding titlebar in the app itself. Very handy to get the maximise button back in the app window itself -thanks! EDIT - what would be really useful is the ability to change the maximise (and other buttons) symbol to something else rather than the box/square symbol so you can get it to match your system theme - even if you were able to do this manually by copying in an image yourself to use as the symbol? Or is that already possible?

Yes, so this finally restores what was taken away by dumb computer jocks in Win10.
BUT it still makes the symbols hard to see. Why can't they be white against back, rather than black on black?

perfect for kde users that want a windows-like experience where the titlebar and border are hidden

Parfait pour Gnu/linux avec kde

It is very useful under KDE and other window managers systems. Obviously not in Windows or MacOS.

Great!

Great.

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 ...