Since Firefox is a Gtk-based application, have you set a dark theme for Gtk applications?
I think in KDE the option is in Settings > Application Style > Configure GNOME/Gtk Application Style.
If you select a dark theme, like Breath-Dark for example, Firefox should pick up that setting and display certain websites in a dark mode.
The same goes for the “internal” pages of Firefox, like about:preferences or about:config for example.