It depends on your Gtk theme, I think. If it's marked as dark, then Firefox will pick that up and apply the CSS accordingly. I use Yaru-dark. You can also forc

It depends on your Gtk theme, I think. If it's marked as dark, then Firefox will pick that up and apply the CSS accordingly. I use Yaru-dark. You can also force Firefox, via an about:config setting to act as if you were using dark mode.