I'm running Ubuntu 14.04, Mozilla Firefox 29.0 for Ubuntu Canonical 1.0.
After installation I've watched some YouTube videos and clicked fullscreen, and the popup 'allow full screen?' appeared. By misclick I've clicked 'Don't allow' and 'never ask again'.
Where is this information stored? How can I reverse it? It's very annoying since I can't view embedded YouTube videos in fullscreen until I repair it.