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?
I guess this is really a bug in KDE/Plasma’s themes, then? The window controls in the GTK3 theme should have the same look as the window controls in the icon theme.