Is there a particular reason why Firefox does not support playback of MP3 files in <audio> elements, only Ogg format?
Is it a licensing issue?
Are there any plans made for a possible future implementation?
Is it possible to develop an addon to support MP3 playback in <audio> elements?
Licensing issues: HTML5 video and H.264 – what history tells us and why we’re standing with the web and Mozilla defends Firefox's HTML5 support for only Ogg Theora video (despite their titles, they both also talk about MP3 licensing, albeit briefly).
All you can do is fall back to Flash and play them through that.