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.

Source not supported usually means that Firefox can't use the built-in HTML5 media player to play a media file (video or audio tag) likely because only MP4 files are served.

• https://developer.mozilla.org/en/Media_formats_supported_by_the_audio_and_video_elements