Some problems with Flash video playback can be resolved by disabling hardware acceleration in your Flash Player settings. (See this article for more information on using the Flash plugin in Firefox).

To disable hardware acceleration in Flash Player:

1. Go to this Adobe Flash Player Help page.
2. Right-click on the Flash Player logo on that page.
3. Click on Settings in the context menu. The Adobe Flash Player Settings screen will open.
4. Click on the icon at the bottom-left of the Adobe Flash Player Settings window to open the Display panel.
   
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5. Remove the check mark from Enable hardware acceleration.
6. Click Close to close the Adobe Flash Player Settings Window.
7. Restart Firefox.

This Flash Player Help - Display Settings page has more information on Flash Player hardware acceleration, if you're interested.

Does this solve the problem? Let us know.

You can check for problems with current Shockwave Flash plugin versions and try this:

• disable protected mode in the Flash plugin (Flash 11.3+ on Windows Vista and later)
• disable hardware acceleration in the Flash plugin

See also:

• http://kb.mozillazine.org/Flash#Troubleshooting

You can also try to set the Flash plug to click-to-play by setting the permissions for Flash to "Ask To Activate" in "Firefox/Tools > Add-ons > Plugins".
That way you can only activate the Flash plugin when you need it.