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.
The following pages use HTML5 to embed audio and video in supported browsers such as Firefox and SeaMonkey 2, without needing a plugin (otherwise, Flash is used). Direct links to "open media" files (OGG and WebM) are also included.
For more information about HTML5 audio and video and open media files, see Viewing HTML5 audio and video in Firefox at support.mozilla.org