1) visit the amazon mp3 store and find an album with in-page mp3 samples 2) hit a play button Firefox will download an .m3u playlist file which it then dumps off to the music app built into metro. This app can't play the music either.
Pretty sure this is a "we don't have Flash" problem.
Confirmed that if I disable flash desktop Firefox works in the same way. If it was just trying to play the file with an audio tag I think it would work through WMF. I think this is WONTFIX.