You don't need a plugin to get the menu bar back, you can just do (tap alt) View->Toolbars->Menu Bar, at least in FF56. I don't know for certain if that option still exists in FF57, but I hope it does.

I've been turning the menubar back on in Firefox for ages now. It's really dumb to hide it IMHO because they never used that space for anything else. It was just useless blank space.