The version 3 branch of mplayerplug-in went stable recently in portage, and one of the things it does is handle a lot more MIME types, it seems.
I have my Mozilla ‘Helper Applications’ setup to send any audio/x-mpeg-url types straight to audacious, but mplayerplug-in overrides it.
The fix is easy though. Edit ~/.mplayer/mplayerplug-in.conf and add enable-mp3=0 Then, remove ~/.mozilla/pluginreg.dat so it will recreate itself next time you open the browser. And yer done.
It’s worth noting that the project has some good documentation on its website that explains all that.