First make sure you have downloaded and run the WMP plugin installer available from http://port25.technet.com/pages/windows-media-player-firefox-plugin-download.aspx

After running the WMP plugin installer search you system for a file called np-mswmp.dll, if you have the release version of Firefox installed you are likely to find it at C:\Program Files\Mozilla Firefox\plugins

Copy this file and place it in the plugins folder for your Firefox 4 beta installation. By default there will not be a plugins folder so you will need to create one. For a fresh Firefox 4 beta 2 installation, you need to place the file here C:\Program Files\Mozilla Firefox 4.0 Beta 2\plugins

hi! i cant find the np-mswmp.dll what can i do ?

Make it easier. C:/Program/Mozilla Firefox (the old installation) and copy the entire folder "plugins". Now go to the path of the Firefox beta installation and paste the folder. All the old add ons will work. Bye

WHen I ran the WMP plugin installer it complained that it couldn't find the 'Plugins' directory. In Firefox 4b8 there is a plugins file not a directory. I tried reinstalling firefox 3 in another directory and was then able install it for 3 but couldn't figure out how to get it to 4.

The plugins folder does not exist in a new installation of Firefox 4 beta, you need to create the plugins folder inside the installation directory for Firefox 4 beta if you want to add a file to it.

np-mswmp.dll
That shows as a Microsoft@ DRM file on WinXP - Firefox. I think the one you need is npdsplay.dll, which can be found here C:\Program Files\Windows Media Player\npdsplay.dll for the Windows Media Player Plug-in Dynamic Link Library. I might be wrong, but the same may be true for Vista and Win7.

You shouldn't to need add a \plugins\ folder and copy the DLL file to that folder. Sure it works, but Firefox should be able to "read" the location of that DLL file thru the Windows Registry. Plugins haven't been allowed to "drop" plugin files into the \plugins\ folder as of Firefox 3.6, they have to "do it" thru the Registry. Fortunately for users, we can still "drop" plugin files into that folder if we need to - but as mentioned, that folder has to be created by the user for 4.0+ versions.

npdsplay.dll is the old (classic) WMP plugin. np-mswmp.dll is the new WMP plugin that you need. The DRM plugins are extra in case you need to handle DRM media content from within Firefox.

Did you try to do a search for the np-mswmp.dll file on your computer?

See also http://kb.mozillazine.org/Windows_Media_Player#Missing_plugin

Problem: Installs of the latest WMP plugin into FireFox 4 RC on Win7 always silently fail.

Solution: Uninstall (using Control Panel > Programs and Features) the WMP plugin for FF which had been installed previously for an earlier version of FF. Then install the latest WMP plugin from port25 link above.

I am using Windows 7 64 bit. After trying all the methods mentioned above, including from mozillaZine, I still can't play any windows media files. Could someone please help?

Try to install the Windows Media Player from Microsoft Windows components. In my case it has not been installed and "Program Files\Windows Media Player"-folder was nearly empty. Check it.