Mp3s open in Google play after tapping always, have already reset all app preferences. What now?
In the Firefox address bar, enter about:config.
search for download.preferred.
After the search had filtered some items, You will see a preference named browser.download.preferred.audio/mpeg whose value is probably com.google.android.music.
Tap the Reset button at the lower right corner of this preference.


Future tip:
To see preferences you modified yourself, enter about:support, and check under Important Modified Preferences.