From MozillaZine Knowledge Base
You can test your plugins to see if they are working by visiting the pages listed below.
The embedded media on these pages require a plugin:
Other media tests
Note: You need a plugin for embedded media on webpages but direct links to media files can be opened in a "helper" application, if no plugin is set to handle the MIME type. Also note that Firefox and SeaMonkey are able to handle certain media types without a plugin or helper application, including ogg, webm (see below) and, starting in Firefox 21 on Windows 7, downloaded mp3 and certain mp4 files via Windows Media Foundation (see bug 799315).  
The following pages use HTML5 to embed audio and video in supported browsers such as Firefox and SeaMonkey 2, without needing a plugin.
For more information about HTML5 audio and video and open media, see Viewing HTML5 audio and video in Firefox at support.mozilla.org