Did you try to run the installer as Administrator?

You can try to search for the WMP plugin (np-mswmp.dll) and copy the plugin to a folder where Firefox can find it (e.g. create a %APPDATA%\Mozilla\plugins folder).

• https://developer.mozilla.org/en/Gecko_Plugin_API_Reference/Plug-in_Basics#How_Gecko_finds_plug-ins
• http://kb.mozillazine.org/Windows_Media_Player#Missing_plugin

I've installed the plugin 5 times and that dll isn't anywhere to be found. Plugin will not show in the pluigins list either. Thanks for the suggestion anyway.

I ran into this problem today with FF 18.0.1 under Win7 64-bit.

I downloaded the WMP FF plugin installation package from Interoperability Bridges as it says here: [https://support.mozilla.org/en-US/kb/play-windows-media-files-in-firefox?...].

After installing, I reopened FF but the plugin was not listed. I tried the "Install Add-on from File" option, navigated to the DLL, and it said it could not install the plug-in because it "appears to be corrupt."

I tried a couple more times because one thread said it worked after getting the installation package from a different mirror, but no luck.

I found in this bug report (Klaassen-Comment 14) [https://bugzilla.mozilla.org/show_bug.cgi?id=421820#c14] that copying the DLL to this folder might work:

C:\Users\username\AppData\Roaming\Mozilla\Plugins\np-mswmp.dll

I did, and it installed without complaint. Obviously, it was not corrupt.

See, however, Comment 15 in the same bug report for an alternative.

Someone needs to add this information to the support page on how to install the plug-in.

See also:

• http://kb.mozillazine.org/Windows_Media_Player#Missing_plugin

Great link, cor-el! Thanks!