From MozillaZine Knowledge Base
You can test your plugins to see if they are working by visiting the pages listed below.
Except as noted, the embedded media on these pages require a plugin:
Other media tests
These test pages may contain direct links to media files as well as embedded media. Direct links to media files can be opened in a "helper" application, if no plugin is set to handle the MIME type. Note that Firefox and SeaMonkey are able to handle certain media types without a plugin or helper application, including open media such as ogg and webm (see below) and certain other media file types. See File types and download actions#Built-in support for certain media for details.  
Note: In mozilla22 (Firefox 22, SeaMonkey 2.19) embedded audio won't play on webpages that use the <embed> tag, if the "hidden" attribute is set (bug 890516, fixed in Firefox 23).
The following pages use HTML5 to embed audio and video in supported browsers such as Firefox and SeaMonkey 2, without needing a plugin. These pages may also include direct links to OGG and WebM "open media" files.
For more information about HTML5 audio and video and open media, see Viewing HTML5 audio and video in Firefox at support.mozilla.org