Change default app for mp3 in Firefox
This is the setting of Firefox on Android and not android itself
Follow these steps:


Open Firefox and in address bar type "about:config"
Search For "browser.download.preferred.audio/mpeg" (just "mpeg" does the same)
Press on the reset button
From now on, if you download any mp3 file in Firefox, it asks for available apps capable of playing the file. You can again choose once or set it as default.