1) Try another site; the problem might be on the other end.
2) Make sure you have all plugins and updates for Firefox
3) Try IE7 or IE8 or some other Browser.
4) Closed uneeded windows and applications.
5) Make sure you have about 1 gig free space on th HDD
6) Make sure the CPU is only 2 0r 3 % before you start
7) Make sure you have all the recent Java updates
Make sure you have the player Firefox is trying to call
9) Close some of the startup applications not needed.
10) Make sure there is not an update at the same time you play the video