I've been using Ff for years and v3 since whenever, don't use any download managers. I always notice at the end of the download, but before the Sandboxie recover option opens, the file is being scanned. I always just assumed it was an action built into NOD32. I guess not!
http://www.pcworld.com/article/159123/safe...ml?tk=rss_howto
An added layer of protection maybe however I have my doubts that it's using "your antivirus software" to scan the downloads unless pointed to an actual and official Mozilla article. My doubts are because I use Avast and the scanning engine takes longer to load on average than the "virus scanning" service done by Firefox 3.
I also wouldn't trust Firefox 3 to catch something a traditional antivirus, or modern antimalware program such as MBAM would catch. You should still scan your downloads with the security software you have installed.