User Agent: Mozilla/5.0 (X11; Linux x86_64; rv:9.0) Gecko/20100101 Firefox/9.0 Build ID: 20111122192043 Steps to reproduce: Went to http://www.youtube.com/watch?v=DsaJMhcLm_A&webm=1&html5=1 and tried to enter Full Screen mode. Actual results: Video did not go into Full Screen mode, not even enlarge it. This is new in Firefox 9.0 beta, works well in Firefox 8.0. Interestingly, Aurora and Nightly are not affected as of November 29. Expected results: Video should have gone into Full Screen mode.