Apparently Mozilla is testing a new dark theme in the latest Nightly builds of Firefox. Mozilla made the user-chrome area a comparatively much lighter shade of gray and the contrast between the address bar and the rest of the user-chrome is significantly reduced.
I personally do not like it. I think the shade of gray they chose for the user-chrome is too light for a dark theme and there is not enough contrast between the address bar and the rest of the user-chrome.