Try these two things;

Hello,

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.

Start Firefox in Safe Mode {web link} While you are in safe mode; Press the <Alt> or <F10> key to bring up the tool bar. Followed by;

Windows; Tools > Options Linux; Edit > Preferences Mac; application name > Preferences

Then Advanced > General. Look for and turn off Use Hardware Acceleration.

Poke around safe web sites and see if there is still a problem. Then restart.

I have done this and it doesn't help. Any other tips?

Some added toolbar and anti-virus add-ons are known to cause Firefox issues. Disable All of them.

Start Firefox in Safe Mode {web link} While you are in safe mode;

Type about:preferences<Enter> in the address bar

Select Advanced > General. Look for and turn off Use Hardware Acceleration.

Poke around safe web sites. Are there any problems?

Then restart.

Still have problems after doing this.Could it be my router and/or modem?

If you want, totally unplug all cables and wait 10 seconds. But I don't think it will help.

Start your Computer in safe mode. Then start Firefox. Try Safe web sites. Safe Mode; Free Online Encyclopedia