There has been a change in where Firefox searches for plugins and the plugins folder in the Firefox program folder is no longer scanned for plugins, so Firefox won't find them anymore if they are installed in the plugins folder instead of the "browser\plugins" folder that Firefox now scans. You can set the plugins.load_appdir_plugins pref to true on the about:config page to make Firefox scan the "<install>/plugins/" directory for plugins like the WMP plugin (np-mswmp.dll). You can alternatively move plugins from "<install directory>\plugins\" to "<install directory>\browser\plugins\" (create this folder when missing) to make Firefox find them again. *http://www.ghacks.net/2013/05/15/why-you-may-have-lost-access-to-plugins-or-extensions-in-firefox-21/ *http://mike.kaply.com/2013/04/24/major-changes-coming-in-firefox-21/ *https://support.mozilla.org/kb/windows-media-or-other-plugins-stopped-working