From MozillaZine Knowledge Base
You can see what plugins are installed and enabled by visiting these pages (see also about:plugins):
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:
- Note: Embedded PDF files don't require a plugin and will work in the built-in PDF viewer in Firefox (see bugs 738967 and 855666 for details).
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, WebM and certain other media file types (see below). For more information, see File types and download actions#Built-in support for certain media.  
- Audio Test Files at www.thesixteendigital.com (Links to MP3, FLAC, and M4A files)
- Example FLV Files at mediacollege.com (Links to FLV files)
- File Tests at edmullen.net (includes links to various file types and test pages, including embedded audio)
- Mime/File Types Test Suite at yolinux.com Note: Not all links here are working
- Embedding audio with the Object tag at lineone.net (includes embedded audio for testing QuickTime and WMP plugins)
- Technology Demo and Info page at csumb.edu (includes embedded MP3 audio)
- Flip4Mac Test Page at thirdplanetvideo.com (Links to WMV files)
- Plugins Test Cases Page at www-archive.mozilla.org (The content here is outdated and many of the linked test pages no longer work)
- Viewer Test Page at sprott.physics.wisc.edu (Test links for various file types, including audio and video files)
- SV&V Media Content Index at download.wavetlan.com (links to many different types of audio, video, image, and playlist files)
- "Embed hidden = true not working in Firefox 22" Test page for embedded WAV audio (linked from bug 893377) 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).
HTML5 audio and video
The following pages use HTML5 to embed audio and video in supported operating systems and browsers (such as Firefox and SeaMonkey 2) without needing a plugin.
The above 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