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
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.