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 got the answer in a crosspost I did: https://bugzilla.mozilla.org/show_bug.cgi?id=1967099
It was reported as bug but got confirmed as intended behavior. Nothing anyone can do short of editing the system icon theme