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.
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.
