Please, I'm there two days trying to work in Firefox MP3 / Ubuntu mate in PI3, but did not work I've installed many codec type, extra restricted, gstreamer ugly, gstreamer, rhythmbox and several others and nothing. Already realized setting in about: config in Firefox and nothing too. I reinstalled Ubuntu again and only installed extra restricted and no sound in MP3 Firefox in PI3, works only audio OGG. What do I do? I will be very grateful to anyone who can help me? In Ubuntu Mate 16.04, Chromium works perfectly with mp3 sound too. I can make work perfectly in Raspbian OS, but i should desactive "media.ffmpeg.enable" in about:config in firefox. In this method i using gstreamer codec.