Windows Media Player
From MozillaZine Knowledge Base
Some websites require the use of the Windows Media Player (WMP) plugin for certain content such as embedded audio and video. This article describes how to get the Windows Media Player plugin working on Windows operating systems.
Standard WMP plugin: In Windows XP and earlier, the WMP plugin file "npdsplay.dll" and related plugin files are normally included in the Windows Media Player program folder. The WMP plugin is automatically detected through plugin scanning and will be used by Mozilla applications for embedded media that require the WMP plugin. Important: Microsoft Security Bulletin MS06-006 (February 2006) reported a vulnerability in the standard Windows Media Player plugin file "npdsplay.dll" on Windows 2000 and Windows XP systems, that could result in remote code execution when using non-Microsoft web browsers. The "Security Update for Windows Media Player Plug-in (KB911564)", available from Windows Update or from the download links given in the security bulletin, updates the file "npdsplay.dll" (normally located in the C:\Program Files\Windows Media Player folder) to version 188.8.131.529. If your system includes the standard WMP plugin, make sure that it is the updated version of this file.
New WMP plugin for Windows XP and above: Windows 7/Vista and some versions of Windows XP do not include the standard WMP plugin. Microsoft's Technet division has developed a new Windows Media Player Firefox Plugin (file name "np-mswmp.dll") for Windows XP and above. This new plugin can be installed to solve a missing plugin issue (see below) or to take advantage of its new features (e.g., scripting support) on systems that already include the standard WMP plugin. Instructions for installing the new WMP plugin in Firefox or another browser are given below.
The WMP plugin test pages Thailand Videos at jeffersonscher.com and Windows Media test at vdat.com contain embedded media that require the Windows Media Player plugin. See the Testing plugins article other WMP plugin test pages.
The WMP plugin is not included in Windows 7/Vista  and may also be missing in other Windows systems. You may visit a site that includes embedded media requiring the WMP plugin and receive a message such as Unknown plugin (application/x-mplayer2)  or you may be incorrectly directed to reinstall the WMP program, which is not a solution, since reinstalling or updating Windows Media Player does not add the missing plugins .
If your system is missing the WMP plugin, use one of these solutions:
Installing the new plugin
Firefox and SeaMonkey users on Windows XP and above can install the Windows Media Player Firefox Plugin provided by Microsoft. 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. Since Firefox 4, the "plugins" folder does not exist by default but will be created as needed. If the Firefox installation directory cannot be located, the plugin file "np-mswmp.dll" will be placed in another location such as C:\PFiles\Plugins and you will need to copy it to your Firefox or SeaMonkey installation plugins folder. For SeaMonkey, the file "np-mswmp.dll" can be copied to the SeaMonkey\plugins folder, which you can create if it doesn't exist.  
Important: In current versions (since Firefox 21), the <installation directory>/plugins folder is not scanned by default. If you have installed the new WMP plugin and it is not detected, you can set the preference plugins.load_appdir_plugins to true in about:config. Another option is to move the "plugins" folder" inside the "browser" folder, located inside the Firefox installation directory. For example, in current Firefox versions on 64-bit Windows, the plugin "np-mswmp.dll" will be detected if it is located in the C:\Program Files (x86)\Mozilla Firefox\browser\plugins folder.        .
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 inside the C:\Users\<username>\AppData\Roaming\Mozilla folder on Windows 7 or Vista) and then copy "np-mswmp.dll" to the new "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, 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: This extension has been reported to cause excessive memory usage in Firefox 5 or above. 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).     
Restoring the standard plugin
The standard WMP plugin will work on most Windows versions and Mozilla-based browsers but it may be missing functionality that the new WMP plugin offers. You can restore the standard WMP plugin files that are normally included on most Windows XP and earlier operating systems. To check for these files, type about:plugins into the location bar. There should be an entry for Windows Media Player Plug-in Dynamic Link Library (File name: npdsplay.dll) and two entries for Microsoft® DRM (File name: npdrmv2.dll and File name: npwmsdrm.dll). If you do not see these files, follow these steps:
Using an external player
Content does not play when multiple Windows Media plugins are enabled
Some Windows Media may not play if you have more then one plugin enabled that can handle that content (you can check your about:plugins list for enabled plugins and content handled). Make sure only one Windows Media plugin is enabled in the Add-ons Manager Plugins list, preferably the new "Microsoft® Windows Media Player Firefox Plugin", and disable the other plugins that handle Windows Media; for example, disable the standard "Windows Media Player Plug-in Dynamic Link Library" (included with Windows XP) or the VLC Media Player Mozilla plugin.  
Note: The new "Microsoft® Windows Media Player Firefox Plugin" is preferable to the standard "Windows Media Player Plug-in Dynamic Link Library" included with Windows XP because the standard WMP plugin doesn't handle direct media links very well and may open a blank page if the media file (e.g., an .asx file) is not associated with any player. 
Video is invisible or disappears when you click a control
Firefox crashes on exit if the WMP plugin has been used (Windows 2000)
WMP6 used even though WMP9 or later is installed
If the player controls look like they are from an older version of Windows Media Player:
"Cannot create DirectShow Player"
Nothing happens when pressing Play
The players in the test links here have the status bar - it tells you what the player is doing. This is not always the case. Sometimes when you hit play on a player without a status bar, it seems like nothing happens - in fact, the player is probably just downloading enough of the file to start playing it (buffering) and if you give it a moment, it will start.
Illegal operation in Windows Media Player plugin
Illegal Operation in Plugin Windows Media Player Plug-in Dynamic Link Library The plugin performed an illegal operation. You are strongly advised to restart Navigator.
Windows XP Home N or Windows XP Professional N
Because of a recent decision by the European Commission, Microsoft have been required to offer these new European versions of Windows XP Home/Pro in addition to the standard verisons, with Windows Media Player and all built-in media playback abilities removed - including the Windows Media Player plugin. The N stands for 'Not with Windows Media Player'. Because of the scope of the changes, you cannot simply install Windows Media Player to get full functionality back. Microsoft have released an update to convert Windows XP N to regular Windows XP (by restoring all the missing files). Users of Windows XP Home/Pro N will need to install this update to run the Windows Media Player plugin. This update is also available on Windows Update. This cannot be undone. After installing this update, the only way to revert back to Windows XP N will be to re-install Windows.
Player appears and status bar says Ready but nothing happens when you click Play
If the WMP plugin console appears and the player status bar says 'Ready' but nothing happens when you click Play and no error messages appear, this can be caused by internet connectivity issues.
Internet Explorer also needs to be able to connect to the internet for the Windows Media Player plugin to work:
This problem can also be caused by DSL/cable providers using Network Address Translation (NAT) not forwarding UDP packets properly. The solution is to disable UDP in Windows Media Player:
It has also been reported that reinstalling/upgrading/downgrading Windows Media Player can solve this.
WMP thinks it is always offline or returns an error that it cannot play any items in the playlist
You may receive an error that WMP needs to connect to the internet, or a message similar to "Windows Media Player cannot play any items in the playlist. To find information about the problem, click the Now Playing tab, and then click the icon next to each file in the List pane." when attempting to connect to online content such as internet radio . The instructions given here may help resolve the problem. Also, some sites may require cookies for the content to play so make sure that cookies are not being blocked. Internet firewall or other security software can also block WMP so check your firewall settings and make sure that WMP is allowed access to the internet. If you are running Zone Alarm, set it to Medium security, or go into the Firewall settings page and hit the "Custom" settings button, enable the "Allow outgoing TCP ports" option, then specify "554, 1755" as the selected ports. 
Crashes or missing WMP controls while trying to play embedded WMV files
Your browser may crash or, if the video does play, the WMP controls (play/pause/stop) or right-click options may be missing . This can happen when the VLC Media Player is installed and its own browser plugin takes precedence over the WMP plug-in, causing a conflict. To resolve the issue, remove the VLC plug-in file npvlc.dll which is usually located in your Mozilla browser's installation directory plugins folder (e.g., C:\Program Files\Mozilla Firefox\plugins\).
Player does not appear but audio sometimes still plays
If the Adblock extension is installed, the OBJ-TABS setting may hide the WMP plug-in. If you find this happens to you, disable Obj-Tabs in your Adblock options. If that doesn't work, try installing the new WMP plugin or restore the standard WMP plugin, as described above and place the plugin file(s) in your browser's plugins folder. The VLC Media Player plug-in has also been known to do this - try removing the VLC plug-in (see above).
Windows XP Media Center Edition
Windows XP Media Center Edition does not come with the Windows Media Player plugin so you will need to add it (see the Missing plugin section, above). If these solutions don't work, the update for Windows XP N (XP with no media playback ability (see Windows XP Home N or Windows XP Professional N, above) is reported to to add back the missing files and settings that allow Windows XP Media Center Edition to support the WMP plugins . Warning: this update cannot be uninstalled. The only way to undo it is to re-install Windows, so attempt at your own risk. The update is here.
Some sites are coded to invoke Windows Media Player through ActiveX. An ActiveX plugin is available for some versions of Mozilla products, but it's generally not recommended.
For Mac and Linux users