The Firefox OS Music app can play most popular audio files. This article explains how to transfer music from your computer and, for versions 1.1 and higher, how to download it from the browser or receive it over Bluetooth.
Load a music file in the browser, long-tap on it and then choose Save audio.
Your Firefox OS device can receive files over Bluetooth – just make sure Bluetooth is turned on and your device is visible to others.
The Music app supports these audio files (details):