Thanks for your reply. It seems that the blocking was caused by some url specified in my "Site Blocking" list that I build over time(some of them related to web ads), which prevented the videos to start. I completely forgot about this list because in the latest versions of Avast!, it is so well hidden:). The clips are still NOT starting with Firefox(only with IE) but I doubt Avast! has anything to do with it so I take back everything I said in my previous post.