Thanks to the work of a number of helpful people I now know how to configure
KDE plasma to allow Firefox to work in dark mode and support the CSS prefers-color-scheme
feature.
In KDE Settings search for GTK. This brings up the GTK Application Appearance panel.
In here you can choose the GTK2 and GTK3 themes.
Adwaita-dark is available for GTK2 but there are no GTK3 dark themes listed.
I cannot find a GTK3 dark theme in the Fedora 30 repos.
I had to install a dark theme from the Choose Theme button - I use Mojave-dark.
Once I had done that Firefox come up in dark mode.
Am I correct in thinking that there are no packages with a dark theme for GTK3 apps?
Barry