Status Update 2/26/08 - Itunes issues.
Since the release of Itunes 7.6, there is a known web streaming issue using Firefox 2. Upgrading to Itunes 7.6.1 will most likely resolve the issue.
To listen, you'll need a player. You might already have one installed.
If not, there are a number of FREE PLAYERS, including: ItunesWinampWindows Media PlayerRealPlayer