adding to the review of mr.buerger, which i second, https://bugzilla.mozilla.org/show_bug.cgi?id=861090 is an open bug report for this issue but it hasn't seen much attention yet. BTW, someone has asked about it in mozilla support forum: https://support.mozilla.org/en-US/questions/959542
btw, does it work the same on other audio and video formats including FLV ?
It switches off all of the options that cause firefox to play media files (mp3s) instead of downloading them. It does this through a toolbar button to turn on/off the config options for media files. That can be useful if you need to change it often. Having it in a preferences window would be preferable to me.
OR, firefox could actually have a reasonable system for managing how file/MIME types are handled so that add-ons like this wouldn't be necessary. Instead of setting the MIME type for audio files to download and have it ignore that completely and open in their own built-in media player unless you dig down to the potentially dangerous about:config settings.