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.
I have a few song I always try on these kinds of sites; Flash & the Pan - Hey St Peter, The Bolshoi - Away, and Future Bible Heroes - Blond Adonis. This site got two out of three, which ain't bad... (Haha, bad joke?)