Make sure your using latest version of Adobe Flash or Download from the below link

• http://get.adobe.com/flashplayer/

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Please check if all your plugins are up-to-date. To do this, go to the Mozilla Plugin Check site.

Once you're there, the site will check if all your plugins have the latest versions. If you see plugins in the list that have a yellow Update button or a red Update now button, please update these immediately.

To do so, please click each red or yellow button. Then you should see a site that allows you to download the latest version. Double-click the downloaded file to start the installation and follow the steps mentioned in the installation procedure.

For YouTube, you normally need Flash Player.

1. From the following page, save the installer for other browsers.
   • Direct downloads of Flash Player for Windows | Adobe Support
2. Exit all applications.
3. Run the installer for other browsers.

Are you referring to the following page, or another? These videos should play without any additional configuration, especially if you didn't install any add-ons or modify any settings.

• http://www.mozilla.org/firefox/features/

Can you play the videos on these pages?

• http://ie.microsoft.com/testdrive/graphics/videoformatsupport/default.html
• http://www.quirksmode.org/html5/tests/video.html