Try this, http://www.youtube.com/html5 http://www.youtube.com/html5 and if this will not play then it is most likely an issue with Firefox; try installing Chrom

Try this, http://www.youtube.com/html5 http://www.youtube.com/html5 and if this will not play then it is most likely an issue with Firefox; try installing Chrome and see if that resolves the playback issue. I was experiencing the same issue in Firefox 3.5.5