Reply To: Firefox Media
I think I found something here.
Starting in Firefox 21 on Windows 7 and above, downloaded MP3 files (also MP4 files with the H.264 video codec and either AAC audio or MP3 audio codec) are also automatically played. Firefox Options -> Applications settings for those MIME types are ignored, unless you go to about:config and toggle the preference media.windows-media-foundation.enabled to false
So try entering about:config in your URL/address bar, enter the key media.windows-media-foundation.enabled for the Filter, then, assuming that key shows up, change its status to false.