Windows Media Player Firefox Plugin first needs to download this software
http://port25.technet.com/videos/downloads/wmpfirefoxplugin.exe
And you can obtain the object through window.document[mediaplayername] and perform operations similar to IE.
However, there are some differences in specific properties and methods. You can refer to the documentation on msdn:
For details, please refer to http://msdn.microsoft.com/en-us/library/bb249579(VS.85).aspx