When a plugin is needed to display this content (wma, mp3 etc) in Firefox
If you tried to play an embedded audio or video file in Firefox, only to receive the message that “A plugin is needed to display this content”, most probably you need to install (or repair the installation of) the Firefox plugin of Windows Media Player.
You can download it now if you don’t have it. Run the executable and select the “change” option. Restart Firefox and visit again the page with the media file you tried to open. Firefox will give you a prompt to activate the plugin once or permanently. Select the option you prefer and enjoy!