From MozillaZine Knowledge Base

Plugins can be tested by visiting the pages listed below.

• Java test page at java.com (More Java test pages here)
• Test Adobe Shockwave & Flash Players at adobe.com
• Adobe Flash Player test page at adobe.com
• Flash video with sound (YouTube Help Center)
• Embedded PDF at plugindoc.mozdev.org
• QuickTime - Movie trailers at apple.com
• Windows Media Player plugin - WMV at plugindoc.mozdev.org
• Plugins Test Cases Page at www-archive.mozilla.org

Other media tests:

• RealAudio and RealVideo Test Clips at service.real.com
• Direct link to Adobe PDF file (1.4 MB)
• Embedded sound test pages
• Media Tests - Audio and Video
• Vorbis.com Direct links to audio files (OGG, WMA, RM, MP3, MP4, WAV)
• Mime/File Types Test Suite Note: Not all links here are working

Check for plugin updates:

• Plugin Check at support.mozilla.com