Avast supports pop and smtp scanning even if you will access them in Mozilla. See discussion - http://forum.avast.com/index.php?board=2;action=display;threadid=6677;start=msg52076#msg52076
With regards to AVG free, I don't think it will scan the in/out messages until you open or move the message. The resident shield of AVG should protect you.
MY subscription to Norton will expire soon (came with 'puter). I am looking for a good free antivirus that works with Mozilla email (scanning incoming and outgoing). Does AVG or Avast scan Mozilla email and which one is the better of the two?