I was watching https://www.youtube.com/playlist?list=PLZHQObOWTQDPD3MizzM2xVFitgF8hE_ab and was annoyed by the presence of youtube controls and video title in full screen mode, when you stop the video.

Turns out, there is a plugin to disable them: https://addons.mozilla.org/en-US/firefox/addon/hide-youtube-controls/