Do you have mozplugger installed? That might help. Also, if your machine is low on resources, using mplayer-nogui, instead of mplayer, can help (it uses less resources, I think).This may be a very old FAQ, and if so, my apologies. For several months now, I have been unable to watch video streams from most sites (including e.g. cnn.com) using mozilla/firefox + mozilla-mplayer. The stream seems to connect, plays for about 1/3 of a second, then stops, then starts all over again, does the same thing.
Mark