A FirefoxOS app for set a custom ringtone (ogg file) on Firefox OS > 1.2
This is a "system" app, so you won't see an icon on the homescreen when you install it.
After installing, open the Settings app and click on Sound.
Then tap to change your ringtone and choose "Ringtone Picker".
Allow access to the sd card and choose your ringtone!
To uninstall the app use Settings -> App Permissions.