There seems to be an issue with Firefox 18.104.22.168 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.
iTunes Top Downloads -- December 11, 2011 This is a moment in time snapshot of the current top downloads from iTunes. We take a snapshot each week at the same time and post it here. To preview the songs, click on a link.