New Black Tab works well for me. Seems like all such add-ons take a fraction of a second for the tab to go black -- which means there is a flash of white before the black happens. This add-on generally has a shorter flash of white, but it's still there. I've been using Chrome because the flash of white is almost imperceptible; I'm thinking about going back to Chrome. It seems there is some aspect of FF that is slower, and the add-ons cannot make that go away.