In Firefox 42.0 on Mac OS 10.9.x I can not get a proper fullscreen mode. There is alway some left-overs like the URL or tabs visible. This is pretty annoying. There have been suggestions how to fix this e.g. with right click "Symbolleisten ausblenden" - this just has no effect.

I think the whole think is full unintuitive and there should be a much better userinterface for this with something like "active corners". So the full screen mode should have proper full screen "full" like in all pixels ...