YouTube was one of the first web portals that can display video in HTML5, without having to install any third party plugins. Currently YouTube videos are available in HTML5, allowing playback in high resolution, but this resolution 1080p FullHD (1920 × 1080 pixels) is not installed in Firefox by default.
To activate the option in 1080p in YouTube to play Full-HD in Mozilla Firefox (Linux, Windows and Apple Mac) need additional browser settings, parameter mediasource change false to true.
Enter in the browser address bar about:config, then in the Search box, type media.mediasource.enabled and double click on it to false:
You should get as in the bottom picture:
Now you can watch YouTube videos in 1080p high definition, of course if it uploaded the owner of the video.