Problem: when opening a link which happens to be an mp3, Firefox and QuickTime collaborate to play it in the browser instead of letting you download.

Solution, maybe:

1. install the MIME Edit add-on (you already know you have to restart firefox, and all that)
2. in that add-on, add an audio/mpeg MIME type with an extension of mp3, telling it to save to disk (you'd think this would work, but it doesn't)
3. go to preferences (or options), then the content tab, then the "Manage..." button down near "File Types"
4. there should now be an MP3 file type. tell it to save instead of opening quicktime.

This appears to work, but then the MP3 file type disappeared, so I'm not sure if it'll stick. You'd think Firefox would make it easier (or at least possible) to change file associations for plug-ins....