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?
Not sure if it’s up to date with the most recent change but I think this link has what you are after. I also prefer separate title bars and have rolled back to a more classic style so mine doesn’t seem to be affected by this issue.