(Since no one replied, felt like I should say somethings.)
EDIT::
--Realized some things with Macs ... -_-
**Did you run a Mac software update? Those can be really important (but I'm not sure how they interact with malware).
--Being a technophobe, I don't think you would be running Mozilla Firefox. Once your issue is solved (hopefully, Avast! is enough, or else I doubt many of us know where to go ... except for redoing your hard drive, which you don't want to do), get yourself a copy and install Ad-Block + NoScript on top of it.
--end edit--
You probably aren't receiving fast responses (like avast! forums usually does) since you are in a Mac environment. Different anti-malware programs, different behaviors, and different everything (even avast! for Macs is a bit different).
Alright, what I can suggest for you (sorry, its not much ...):
--Update Avast!
--Setup Avast! to run a boot-time scan (here are some visual directions; not sure if they are the same for Macs:
http://www.schmahl.net/cr/avastbootscan.htm)
--Try to quarantine/ delete those files
**Is there a way you can revert these changes? Like a backup/ system-restore-like behavior on Macs? Time Machine ??
Report back ASAP.
Plus - I thought macs were supposed to be virus free.
--Now you know its not true

. One of the reasons why I hate the Apple philosophy and business structure:
http://gizmodo.com/5040593/cranky-windows-guy-apples-iphone-bugs-stopped-me-from-switching-to-a-mac