Firefox is a web browser made by mozilla, but can be installed in addition to mozilla. They are not the exact same browser.... reading aroudn the mozilla site will explain the differences.
Red had intentionally removed mp3 support from their os, because they are bastards
All you need to do to get it back is to uninstall their media players, and go download the origional versions from their respective websites, and install from souce. a bit of RTFM, but nothing too difficult.