Hello Tom, I'm sorry you're having trouble. Make sure that Shockwave and Flash are enabled in the Add-ons menu. They are under Plug-ins. You may also find many other plug-ins as well, however, only activate them if you need them. If the problem persists, try restarting Firefox with Add-ons Disabled by clicking the Menu button in the top right, then by clicking on the '?' button at the bottom of the menu. From there, select "Restart Firefox with Add-ons disabled..."

Shockwave and Flash are enabled. Still won't work. When I select "Restart Firefox with Add-ons disabled", that works, videos in Yahoo run fine. If I restart Firefox, I'm back to the same problem, since Add-ons come back. No way of knowing which Add-ons are causing the problem.