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.
If anyone here is into blues/jazz/jam stuff, check out the website of Mountain Jam. They have a streaming media player with tons of great tracks. I've been hooked on this all week so far, and I'm not even close to done with it.