Microsoft introduced the Dark theme for Windows 10 systems as early as 2015. Although most UWP apps support this setting, Win32 applications are still improving, and Microsoft's own resource manager has just recently acquired the Dark theme. Now Mozilla's Firefox is the first to embrace this setting, and in the latest Nightly version it can follow the Light/Dark theme settings in Windows 10 to adapt to changes.
Naturally this feature is only available for Firefox's default theme, and custom themes are still not supported. The Nightly version is not suitable for regular users, but if you want to test the new features of Firefox, you can try it out.