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?
Yup, does that break something in the newest update?
Firefox is taking that icon from your GTK theme. And that’s the maximize button in the Papirus theme. So this is intended behavior.
You’d have to modify the theme or tell Firefox to use a title bar to fix it.
I'm surprised all other GTK apps manage to use the plasma theme defaults. But it gives me enough to do some more research. Thank you very much!
Plasma has a not so bad GTK integration which allows GTK apps to use plasma defaults. Also, GTK apps that weren't written by Gnome also have some good integration with other desktops. Only those written by Gnome are hardcoded to use only GTK styles.