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. Grand Theatre Vol. 2 (The W at Amazon) Old 97's (New West Records) 2. Space in Time [Gold CD] Ten Years After (Audio Fidelity) 3. Drums Between the Bells [Limited Edition] Brian Eno (Warp Records) Currently not available 4.