I'm running KDE Plasma, with the latest Firefox and have the titlebar turned off in the customize settings. When I set Firefox to use the "System theme - auto" it correctly uses my window decorations from the GTK theme I set in KDE's settings (top). Any Firefox theme I apply changes the window decorations to the below pic. Is there a way I can edit a theme to respect my system defined window decorations, or else I guess I'm looking for how edit the theme itself so I can define what decorations to use manually.
If you’re using KDE I advise using Waterfox. Waterfox supports more window decorations on KDE than standard Firefox.