If you untick the core_media_enable_mp3 setting in the Media Embedding seccton of Admin (help text: "Files with extension *.mp3. Plays audio using Flowplayer, requires Flash plugin.") and have the core_media_enable_html5audio ticked (help text: "Audio files with extension *.ogg, *.aac and *.mp3. Used primarily for mobile devices. (Format support depends on browser.)") then, unlike Safari/Chrome/IE9+, Firefox will not have audio tags created for it.
This was done since Firefox didn't support mp3, but now it (mostly) does:
As I type that page seems slightly out of date as my version 26 on XP plays MP3 files.
The code that does this is here:
Possibly because of this same code, the files that you upload into Moodle (as opposed to linking to them) get played in Firefox but are treated like video files (and so take up much more space than necessary).
I'd suggest that with this change, the default in Moodle should be to use HTML5 audio rather than Flash, but maybe that's another bug to be filed once this one is done.