Firefox seemingly very recently shipped their own titlebar controls buttons, which worsens even further the lackluster OS integration. In the screenshot you see my regular control buttons on the window to the left (default KDE Plasma theme) and the new custom buttons Firefox is serving now.
Would anyone know how to undo that change in about:config or anywhere else?
Yes, but IDK of any other way to change the buttons ISOmorph was asking about. The only other way is to find and change/remove them in FF source code and recompile which needs a supercomputer to do, otherwise it may take days to recompile. I assume he doesn't want to waste days on recompiling, so that's one way to change things. With GTK3-NOCSD (mostly + some other customizations) my title bar always looks the same way, no matter what changes Mozilla make to it: