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?
What you are referring to are the window decorations.
Apart from Linux Mint, Firefox almost always uses client-side decorations. What you are showing here is still client-side.
It is just that Mozilla recently enabled vertical tabs option for everyone, so the top bar is now slightly smaller than before. You can disable vertical tabs easily by searching in the settings.
Disabling vertical tabs didn’t help. But thanks for trying