It plays on Windows 7 (in a new window).

If you open Firefox > Preferences > Applications, then search for mp3, does the QuickTime plugin show up as the application to play it?

Thanks for responding so quickly. Under Content type, I have a number of lines that say QuickTime but I can't read the whole line and can't figure out how to open up the box to access that info. I have about 12 lines of QuickTime Player Document and they say application, audio, video, etc. after them. The action column says Use QuickTime Plug-in in 7.6.6.

Does that give you anything to go by?

If you float your mouse over an item, after a moment you should get a tooltip with the complete information. (Yes, that is a hassle.)

To better zero in on the podcast, type mp3 in the search box and pause while Firefox filters the list. You should end up with two items. If nothing comes up for mp3, try mpeg instead. If you don't have audio/mpeg and audio/x-mpeg matched up with the QuickTime player, then you may need to adjust the QuickTime options.

On Windows, you do that by opening the player from the menus, outside the browser, going into preferences, and looking for MIME type settings and making sure QuickTime is designated to play mp3 or mpeg audio. I don't know whether that is applicable on the Mac.