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.

Don't confuse the Shockwave Flash player and the HTML5 media player as both are different media players. Firefox 33 will try to use the HTML5 media player on YouTube because that website supports canPlayType to detect if the browser is capable of playing a specific media file (codec) directly.

See Detecting Playback (HTMLMediaElement.prototype.canPlayType):

• https://developer.mozilla.org/Apps/Build/Audio_and_video_delivery/H.264_support_in_Firefox

See also Windows Media Feature Pack for Windows 7 N and for Windows 7 KN:

• http://support.microsoft.com/kb/968211
• http://www.microsoft.com/en-us/download/details.aspx?id=16546

I think the problem is related to the Adobe Flash plugin. I removed it and now I can watch all kind of videos in youtube. However, Firefox asks me to install the plugin. Is this plugin necessary for Firefox to function properly?

You can set the Shockwave Flash plugin to "Ask to Activate" or "Never Activate" if you do not want to use it. Some websites use the Flash plugin for menus or other elements.

You can try to clear the YouTube cookies to see if that makes the site use the HTML5 media player instead of Flash.