In the Firefox Preferences Applications panel on Mac, I see both Wave Audio file and iTunes Document (audio/wav) listed.
Just to clarify, I checked on a computer running Mac OS X 10.6 and the about:plugins list in Firefox 22 (which you can see by entering about:plugins in the address bar) shows that my QuickTime 7.6.6 plugin handles WAVE audio files served with both the audio/x-wav and audio/wav content types.