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.
Sensing an opportunity to move product, Amazon MP3's editors have put together a Top 50 of 2009 (so far) list. I'm reprinting it here in the hopes that it will encourage discussion (but remain pessimistic).