To play Windows media in Firefox, Firefox needs the Windows Media Player browser plugin installed. There are two different versions:
Many Windows XP users will have the old plugin already, but it is recommended that you install the new plugin anyway.
This article describes how to check for and install the Windows Media Player plugin in Windows XP/Vista. XP users may already have the old plugin, but the new plugin is recommended.
To see if you already have the plugin installed:

To see if you already have the plugin installed:

If the entry described above is not present, then the plugin is not installed. You have to download it.
The following link is to a test player that requires the new Windows Media Player plugin. If the player loads and plays, then the plugin is installed correctly: http://plugindoc.mozdev.org/testpages/wmp11.html
Internet Explorer uses the Windows Media Player ActiveX Control to play media, and Internet Explorer will use it to play any media format that Windows Media Player itself can play.
The Windows Media Player plugin will only play the Windows media file types such as wmv and asx, unless the web site is very specifically crafted.
This means that where an embedded mp3, for example, would play in Windows Media Player in Internet Explorer, it would play most likely using QuickTime in Firefox. Windows Media Player does not provide any way of changing the file types the Windows Media Player plugin will play.
For many of these file types, such as midi and mpeg, the QuickTime plugin will first need to be configured to play them - this can be done within its user preferences. Otherwise, you will see a missing plugin alert for QuickTime even if you already have QuickTime installed. For more information, see the QuickTime article.
As an alternative, the IE Media Mimic user script (for use with the Greasemonkey extension) can correct this issue in most cases - embedded media that would normally not play with Windows Media Player in Firefox will be made to.
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Some web pages use JavaScript to build custom player control buttons or to retrieve and display information on the player's current status.
The original Windows Media Player plugin (included with Windows XP home/pro and older) did not support this capability. The new plugin (downloadable for use in XP/Vista) does, but most sites using these advanced functions still specifically target Internet Explorer. So, they won't work even with the new plugin until the page is updated to account for it.
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Some websites embed media in ways that only work in Internet Explorer - either because those who wrote the site do not know how to write code that works in other browsers, or they don't know they need to, or they don't want to.
The aforementioned IE Media Mimic user script can also correct many kinds of IE-only coding.
ActiveX is the system Internet Explorer uses in place of the NPAPI plugin system that most other browsers use. There was a plugin for Netscape-based browsers that grants the browser partial ActiveX support - this would allow Firefox to use the Windows Media Player ActiveX Control and the majority of its functionality, and so work on many otherwise Internet Explorer-only pages.
However, the most recent version only supports Firefox 1.5 and does not work in Firefox 2 or 3. More detailed information can be found in the ActiveX article.
Windows Media Player used to be available for the Mac platform, but is no longer supported by Microsoft. In its place, Flip4Mac is the replacement of choice - it adds Windows media playback capabilities to Quicktime.
Flip4Mac is an add-on for Quicktime, so Quicktime must be installed first. For information on Quicktime, see the Quicktime article.
Download and install Flip4Mac from the official web site: http://www.flip4mac.com/wmv_download.htm
The web site includes more detailed installation and usage instructions.
Windows Media Player has never been available for the Linux platform. mplayer is a popular Linux media player that can play back Windows media. There is a separate browser plugin for it.
The plugin can be usually installed as a package for the current Linux distro used (for example in Ubuntu it is named mozilla-mplayer) or can be downloaded from the mplayerplug-in website and built from scratch - if you choose to do so, please follow the instructions on the author's website.
Based on information from Windows Media Player (mozillaZine KB)
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