I'm using Archlinux and I have the H.264 GStreamer plugin installed (as well as base+bad+good+ugly codecs), but with Firefox 26 I still can't play a lot of YouTube videos when Flash is disabled, for example when I try to play Gangnam Style it tells me "The Adobe Flash Player is required for video playback".
Is that related to advertisements? Would it be possible to develop a plugin or a GreaseMonkey script that would allow to play every YouTube video in HTML5 with Firefox? If yes, does it exist?