Firefox Quantum comes with three themes: Default, Dark and Light. First one enabled by default, you can enable dark or light theme in Firefox browser by visiting the Themes section in the add-ons manager. In the current stable Firefox Quantum 59, Dark theme isn’t dark, the bookmark menu, Page Action menu, the application & the library menus, and the drop downs for URL bar and Search are appearing in white color, something Mozilla has worked on and turned them into black.
Dark theme improved in Firefox 61
FYI, Firefox dark theme isn’t complete, the UX team is still working to apply dark colors to other pages of Firefox such as add-ons, options, library, bookmarks and New Tab Page etc.
If you’re curious to see the progress dark theme has made, switch to Firefox 61
Nightly. Are you a dark theme fan?