Cause: The plugin could be missing, or it might be refusing to work properly.
Solution: Restore the plugin if it is missing, and configure it if it doesn't work properly.
First off, you need to you have Windows Media Player 9 or 10 installed.
The WMP Plugin Test for Firefox You can test the Windows Media Player plugin at http://jrzycrim.sitesled.com/mozilla/wmp/vidtest-LS.html. If the test player works, then the Windows Media Player is installed. If it doesn’t, you need to check that Firefox can detect the Windows Media Player plugin. Type “about:plugins” into the address bar. This will display Firefox’s plugin information. If the following MIME type entries are present under “Windows Media Player Plugin Dynamic Link Library,” then WMP is installed correctly. Note that under the “Enabled” column, all the entries should be “Yes”: application/asx; video/x-ms-asf-plugin; application/x-mplayer2; video/x-ms-asf; video/x-ms-wm; audio/x-ms-wma; audio/x-ms-wax; video/x-ms-wmv; video/x-ms-wvx.
Also, under the following…
“Microsoft DRM File name: npdrmv2.dll
DRM Netscape Network Object”
…the MIME type “application/x-drm-v2” should be enabled.
Finally, under… “Microsoft DRM
File name: npwmsdrm.dll
DRM Store Netscape Plugin”
…the MIME type “application/x-drm” should be enabled.
Note that the “Microsoft DRM” entry with the filename “npwmsdrm.dll” might be called “Microsoft Corp. DRM Netscape Plugin” instead. Now, if any of these entries are missing, the Windows Media Player plugin is not properly installed. To restore the Windows Media Player plugin, look for the files in the Windows Media Player directory (usually C:\Program Files\Windows Media Player), and copy them to the Firefox plugins directory (usually C:\Program Files\Mozilla Firefox\plugins). Then repeat the “about:plugins” step andcheck the files are all there.
If any of the entries are still not present, close Firefox and then install the Windows Media Player Plug-in for Netscape Navigator, which contains some of those files. When installation is complete, repeat the "about:plugins" step and check if the files are all there.
If some of the files are still missing, you can download the individual files from dlldump.com:
Download them to the Firefoxs plugins folder (usually C:\Program Files\Mozilla Firefox\plugins).