Windows Media Player should be found automatically when Firefox starts. There should be no need to copy files into the Firefox plugins folder. If it will not run, this article may help:
http://forums.mozillazine.org/viewtopic.php?t=206216
I have personally only experienced two things that stop Media Play from working in Firefox.
1. JavaScript is not enabled.
2. An older version of Media Player is being called on because of an incorrect Registry entry. This point is covered in the article linked to (see section 3 under 'Configuring/Troubleshooting the Windows Media Player plugin').
Although these points are perhaps worth noting, it doesn't sound as if they are the cause of the trouble in your own case.
If the article does not help, you could maybe copy the Media Player plugin files into the Firefox plugins folder but as already said, it shouldn't really be necessary. Nonetheless, all is worth trying if something doesn't work. The files to copy are the ones beginning with the letters 'np'.
I am running Firefox 1.5 and all of the plugins download as they are needed. The windows media plugin won't download and I am directed to the Windows Media Player 10 download site. I already have 10 and there are no plugin folders in it's program files to copy paste into Firefox's program files. Anyone know where I can find the windows media player plugin?

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