Recently, I realised I can no longer watch YouTube videos in fullscreen. The switch to HTML5 is something that I have missed to notice… Now the videos cannot be shown in fullscreen mode. If you just go to
about:config and set:
true, videos will be shown in almost fullscreen. The navigation and the address bars will still be visible.
I searched a bit and found the True Full Screen in SeaMonkey extension, which will set the above for you and make SeaMonkey hide the location bar, buttons, etc.
Works well. Some additional info here.