A Dark Theme by Firefox user 14666182

A Dark theme with the Bookmarks Toolbar dark as well so it's not harsh on the eyes at night;-)

Just to let you all know, I've tried to find a way to change the background picture for new tabs using "Firefox Color" (which is what I used to create this theme), but I can't seem to find one, so it's blank and not dark.

There are, however, extensions that'll allow you to add your own backgrounds. So, give them a shot.:-)