Open cmd (with Administrator rights) and execute the following command:

regsvr32 C:\WINDOWS\system32\wmnetmgr.dll

I used the "solution" but ended up in a worse situation. How can I reset the command and get back to where I was....??

for people having problems with windows media player plugin and firefox 4 - use this plug-in and all will be well again: http://www.interoperabilitybridges.com/windows-media-player-firefox-plugin-download

thanks for linking the site with the problematic plugin genious

here is the answer, from http://kb.mozillazine.org/Windows_Media_Player

The plugin installer adds the file "np-mswmp.dll" to the Firefox installation directory plugins folder, typically "C:\Program Files\Mozilla Firefox\plugins" or, on 64-bit Windows, "C:\Program Files (x86)\Mozilla Firefox\plugins". If Firefox is not installed or if the Firefox installation directory cannot be located, the plugin will be placed in another location such as C:\PFiles\Plugins [4] [5]. To install the WMP plugin in SeaMonkey 1.x, SeaMonkey 2, Minefield, or in a custom Firefox installation, you will need to copy the "np-mswmp.dll" file to your browser's plugins folder. [6] For example, for SeaMonkey 2, copy "np-mswmp.dll" to the "C:\Program Files\SeaMonkey\plugins" folder or, on 64-bit Windows, to the "C:\Program Files (x86)\SeaMonkey\plugins" folder.

Starting in Firefox 4, the Firefox installation directory does not include a plugins folder. [7] If you need to copy the plugin from another location, you can manually create a new "plugins" folder within the Mozilla Firefox program folder and then copy "np-mswmp.dll" to the plugins folder you created. Alternately, you can create a new "plugins" folder inside the %APPDATA%\Mozilla folder (e.g., inside the C:\Documents and settings\<username>\Application Data\Mozilla folder on Windows XP or the C:\Users\<username>\AppData\Roaming\Mozilla folder on Windows 7/Vista and then copy "np-mswmp.dll" to the plugins folder, so that all Mozilla applications can find it.

Windows 7 users can install the HTML5 Extension for Windows Media Player Firefox Plug-in, currently supported in Firefox 3.6 and Firefox 4, which adds an extension that allows playback of H.264-encoded videos and additionally installs the WMP plugin by creating a C:\Users\<username>\AppData\Roaming\Mozilla\plugins folder and placing the file "np-mswmp.dll" in that location. Warning: At time of writing, this extension is not compatible with Firefox 5 or above and reportedly causes excessive memory usage. If you installed the HTML5 extension and are having problems, open the Firefox Add-ons Manager Extensions list and remove it (the WMP plugin itself will remain installed).