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?
Did you change the gtk theme recently? Firefox follows the gtk3 titlebars, not the qt ones. You would have to change the gtk3 theme back to breeze to have it match again.
If you changed off the default firefox theme, it will also no longer use native titlebar buttons, to make it use native ones with a different firefox theme, go to about:config, search non-native, find the titlebar buttons option, and turn it off.
My GTK Theme is already set to breeze:
Changing the value in about:config had no effect. Thanks anyway though!