After many hours of testing, I solved this problem for this moment. My operating system is Windows 7 (64-bit).

- Close Firefox 4 at first.

1. Install plugin for Windows Media Player normally, from this link: http://port25.technet.com/pages/windows-media-player-firefox-plugin-download.aspx

2. After the installation is created a folder called Plugins in the installation´s directory of Firefox 4

3. Go to the installation´s directory of Firefox 4 and copy the folder Plugins into your profile folder. (The profile folder should contain then folders e.g.: Cache, Extensions ... and other. But one of the folder must be Plugins).

4. After this you can test, if is Windows Media Player plugin functional. By me was this operation successfull and embedded videos in the website can be played now.

(Subjectively: in my opinion has the plugin slower reactions, but videos are played without problems).

Thanks Marian_SK

It works by copying plugins from my default FF3 directory C:/Programs/Firefox to my FF4 directory C:/Programs/Firefox4

also see bug report https://bugzilla.mozilla.org/show_bug.cgi?id=646554 .

Where is this "Profiles" folder you refer to?? I am running the same system and can't find anything like that in the Mozilla Firefox folder group.

Help > Troubleshooting Information > Profile Directory: Open Containing Folder

@aardvark2zz: Yes, it is possible, that problem can be solved in this way too. But I made a fresh installation of Firefox 4 and (after the installation) I removed the installation´s folder of Firefox 3 completely. So now I can´t test your solution. But thank you for your help.

@cor-el: Thank you. The profile-folder can be found very fast and reliably.

In case it helps someone, this is copied from http://kb.mozillazine.org/Windows_Media_Player#Installing_the_new_plugin

To install the new WMP plugin, download the installer file to your computer, then close your browser and run the installer.

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". If Firefox is not installed or if the Firefox installation directory cannot be located, the plugin will be placed in another location such as C:\PFiles\Plugins. To install the WMP plugin in SeaMonkey 1.x, SeaMonkey 2, Minefield, or in a custom Firefox installation, you will need to copy the "np-mswmp.dll" file to your browser's plugins folder. <snip>

Note: Starting in Firefox 4.0 Beta, the Firefox installation directory does not include a plugins folder. If you need to copy the plugin from another location to Firefox 4, you can manually create a new "plugins" folder within the Mozilla Firefox 4.0 program folder and then copy "np-mswmp.dll" to the plugins folder you created. 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 the C:\Users\<username>\AppData\Roaming\Mozilla folder on Windows 7/Vista) and copy "np-mswmp.dll" to the Mozilla\plugins folder so that all Mozilla applications can find it. Windows 7 users can also install the HTML5 Extension for Windows Media Player Firefox Plug-in for Firefox 3.6 and Firefox 4.0 Beta. This extension automatically creates a C:\Users\<username>\AppData\Roaming\Mozilla\plugins folder and places the file "np-mswmp.dll" in that location; however, at time of writing, this extension is not compatible with Firefox 4.

I have Windows 7 (64-bit) + FF4 (release). After all attempts to follow this thread my disk is polluted with copies of "np-mswmp.dll", but it still doesn't work.

I don't know what to tell you. I have no problems with the WMP plugin in Firefox 4 on my Windows 7 64-bit system.

WMP Plugin test link:
http://hg.mozilla.org/qa/litmus-data/raw-file/tip/firefox/plugins/mediaplayer.html

Just as an experiment, I uninstalled the Windows Media Player Firefox Plugin from the Windows Control Panel "Programs and Features" list and then I went to my Firefox 4.0 installation directory, "C:\Program Files (x86)\Mozilla Firefox\", and deleted the "plugins" folder for good measure (it had copies of plugins I didn't need, which were left over from Firefox 3.6 which used to be installed in that location). I also uninstalled the HTML5 extension from my Firefox 3.6 profile (I recently reinstalled Firefox 3.6 in another folder, "C:\Program Files (x86)\Mozilla Firefox3.6"). Then, I removed all copies of np-mswmp.dll (including C:\Users\wymette\AppData\Roaming\Mozilla\plugins\np-mswmp.dll which was left behind) and used the above test link to make sure the plugin was missing (it was).

I then went to the Firefox plugin download page at http://www.interoperabilitybridges.com/windows-media-player-firefox-plugin-download and downloaded wmpfirefoxplugin.exe to my computer, closed Firefox, and ran the installer. That created a new "C:\Program Files (x86)\Mozilla Firefox\Plugins" folder and installed the plugin, which works fine.

I just saw this posted to the mozilla.support.firefox newsgroup in the topic,
Windows Media Player will not install in Firefox 4
Just for the record, the extension that conflicted with Windows Media Player is named: Real Player Browser Record Plug-In 14.0.2
..... so try Troubleshoot extensions, themes and hardware acceleration issues to solve common Firefox problems to see if a Firefox extension is conflicting with the WMP Firefox plugin.

Guys, i had the same problem too,the problem is not firefox 4 or the wmp plugin but the realplayer record browser plugin.Go to add-ons manager in your browser, extensions,and disable the realplayer record browser plugin and the problem is Solved!!!!

Vidjay, Glad to hear that disabling the RealPlayer Browser Record Plug-in extension solved your problem with Windows Media. That extension is known to cause problems and directions for disabling it on Windows are given in the article, Fix problems with RealPlayer add-ons under these Troubleshooting sections:
Webpages are blank after updating RealPlayer
Cannot download videos after updating RealPlayer
Quote:

1. At the top of the Firefox window, open the Tools menu and click on Add-ons. The Add-ons window will open.
2. Select the Extensions panel.
3. Select RealPlayer Browser Record Plugin.
4. Click on Disable.
5. Click Restart Firefox to start Firefox with the extension disabled.

To anyone else having problems, it's also possible that other extensions (e.g., NoScript) are blocking the WMP plugin, so you could try disabling all extensions in the Firefox Add-ons Extensions panel. Other solutions to video and audio problems in Firefox are given here: Fix common audio and video issues

Thanks everyone,

I don't have "RealPlayer Browser Record Plugin" anywhere in addons !

SafeMode does not help ! Only the following works for now for me, see my earlier comment :

"....To get media player files to play I had to copy C:\Program Files\mozilla firefox\plugins\ directory to C:\Program Files\mozilla firefox4\plugins\ (note the 4) directory ...."

Vista 32, anti-virus off

(aardvark2zz) "....To get media player files to play I had to copy C:\Program Files\mozilla firefox\plugins\ directory to C:\Program Files\mozilla firefox4\plugins\ (note the 4) directory ...."

That was because the WMP plugin was already installed in the "default" Mozilla Firefox program folder, where you had Firefox 3 installed. Since you installed Firefox 4 in another location, you had to copy the WMP plugin over to Firefox 4 program folder. That was mentioned in the note I quoted in an earlier post:

Note: Starting in Firefox 4.0 Beta, the Firefox installation directory does not include a plugins folder. If you need to copy the plugin from another location to Firefox 4, you can manually create a new "plugins" folder within the Mozilla Firefox 4.0 program folder and then copy "np-mswmp.dll" to the plugins folder you created.

1. Install plugin for Windows Media Player normally, from this link: http://port25.technet.com/pages/windows-media-player-firefox-plugin-download.aspx 2. Go to program files in windows 7 click on computer/hard disk drive/program files (x86)/Mozilla firefox/ 3. Copy folder plugin 4. Open Firefox 5. Click on help/troubleshooting/information/ 6. In the list under Application Basics find Profile Directory 7. Click on (open containing folder) 8. Paste plugin folder. 9. Close Firefox. 10. Restart Fire fox then visit a site with embedded videos. It will work.

1. Install plugin for Windows Media Player normally, from this link: http://port25.technet.com/pages/windows-media-player-firefox-plugin-download.aspx 2. Go to program files in windows 7 click on computer/hard disk drive/program files (x86)/Mozilla firefox/ 3. Copy folder plugin 4. Open Firefox 5. Click on help/troubleshooting/information/ 6. In the list under Application Basics find Profile Directory 7. Click on (open containing folder) 8. Paste plugin folder. 9. Close Firefox. 10. Restart Fire fox then visit a site with embedded videos. It will work.