I just noticed in FF 3.0.7 that if I list the fonts available for any of the font settings on the Mac, that I get the complete list of fonts. For Monospace, I have Arial (proportional sans serif) and Times (proportional serif) included (among others) in that list. If I look in the Serif list, Arial and Courier are included. Why would that be set up like that? If I have no idea what type a certain font is, those lists are not going to tell me.
At least, for Windows, the list has fonts for that category listed on top, then the whole list underneath.