With no any logical reasons, dully ridiculous ignoring multiple users wish, against any common sense and a simple understanding people’s health requirements,
there’s still IMPOSSIBLE to use system color scheme nor in browsers"about:" area, neither in “mozilla.org” page 
I don’t know ANY acceptable reasons,
WHY this happened and not even showing a single sign to start taking care about users health ???
There is work on making about pages dark. Untill this work is done try https://overdodactyl.github.io/ShadowFox/ and https://addons.mozilla.org/en-US/firefox/addon/darkreader/
Which OS do you run Firefox on?
If your Firefox is not detecting your dark mode preference from your OS automatically for internal about: pages, you can set a preference (ui.systemUsesDarkTheme) to force it.
See the following SUMO thread for detailed steps: https://support.mozilla.org/questions/1305606