You want to watch YouTube videos without installing the Flash plugin.
Normally, the AAC codec from the optional Multimedia Codec Pack is necessary for using the new YouTube HTML5 Video Player.
However, you can trick your Firefox browser into using the free webM codec instead. You can achieve this by simply disabling the browser's GStreamer support. This will make the browser fall back to using its built-in webM codec, which is fortunately supported by the YouTube HTML5 Video Player.
To make your browser play YouTube videos using webM:
For more information about the HTML5 player, see https://www.youtube.com/html5.
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