I found out about this two days ago when a local pal wrote me to ask if I knew any way Firefox could infect a machine. It seems a Vietnamese national he knows who lives here had the problem. I've been following it since.
This is a tale that should teach us not to trust everything. EVERY system in place to protect us can break down. What is remarkable is that this doesn't happen more often.
I had it happen with a very well known download site in 1999. (They shall remain nameless here.) I downloaded a newly posted file and my AV freaked out. I contacted them and that was the end of the problem. However, some 1100 users who had also downloaded the file had to be notified, with red faces all around and many apologies.
Scan your system regularly. It is NOT a waste of time.