I use ViewTube, a GreaseMonket script: http://isebaro.com/viewtube/
ViewTube let you choose what format/quality to play/download.WEBM only is available at low quality on YouTube. If you want high quality, you will have to choose the MP4 format. You can then transcode to WEBM with an application such as Transmageddon, which is in Trisquel's repository. You would lose some quality though.
