We currently support MP3 playback for Windows Vista+ and Android if I'm not mistaken, and support for other platforms is being worked on. Web Audio will support all of the codecs supported by <audio> depending on the platform.
And on Linux, if you enable gstreamer and if your gstream has mp3 support installed. (A lot of ifs, but it is the Linux desktop ...)
Yes, that should be available in Nightly and Aurora: