There seems to be an issue with Firefox 22.214.171.124 and iTunes 7.6 on OS X with regards to M3U files. I've noticed this issue with eMusic as well. To solve this, I've included the URL of the actual streams, and if you enter that URL in Advanced --> Open Stream, that should work. There's also this script, but I haven't had a chance to try it yet.
Note: This problem seemed to resolve with iTunes 7.6.1.
THE TOP SELLERS 1. El Camino (The W at Amazon) The Black Keys (Nonesuch) 2. Very Best Of Neil Diamond Neil Diamond (Sony Legacy) 3. Glee: The Music 7 Glee Cast (Columbia) 4. Lioness: Hidden Treasures Amy Winehouse (Universal Republic) 5. My Heart (U.S.