There seems to be an issue with Firefox 220.127.116.11 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.
Paul Shaffer and the band on Letterman gave him a nice little tribute last night by playing "Rock Box" going into a commercial break. Paul and Dave talked about the times they had Run DMC on the old NBC show...nice touch, I thought.