[ubuntu] webm videos on youtube

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http://ubuntuforums.org – I've been trying to watch webm videos on youtube lately with only partial success. Some of them play alright, while others request Adobe Flash. For example, this video requests flash, and this one plays without. I've verified (using a certain service I am probably not allowed to name here) that both videos have .webms of p360 and p460 available, and have, for the lack of a better option, downloaded the one I couldn't watch on youtube. I use Firefox 15, don't have flash or alternatives installed, do have cookies and scripts allowed, and http://www.youtube.com/html5 shows that I am in the trial (Hardware)