There seems to be an issue with Firefox 184.108.40.206 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.
So much for used record stores. Admittedly, the only place I actually play physical CD's anymore is my car, and even there I can (and sometimes do) plug in my phone and play music directly off of it. Still, I only buy CD's.