Can't play mp3 files using your chosen stand alone player in Firefox........ like Winamp?
See: https://support.mozilla.org/en-US/questions/1110914
(extract copy) (how it's done) (comparison)

To use a stand alone player in firefox - such as WMP Classic
You can open the about:config page via the location/address bar.
You can accept the warning and click "I'll be careful" to continue.
You can set media.play-stand-alone to false on the about:config page.
This way you can choose the preferred player interface.
Codecs