That's in Firefox recent builds. Nice. In particular for dark images with a lot of alpha transparency :-( Now click on the image below to see it standalone. I find the result incredibly disturbing in terms of contrasts. That will probably be a severe accessibility issue for contrast-impaired people. Why was such a change done instead of adding a new checkbox preference to the Content panel of the preferences window "View images over dark background", unchecked by default?

BlueGriffon logo over dark background