Firefox 3 cares a lot about your system's security, so much so that it uses the installed antivirus software to scan the fully downloaded files. Now while this is very thoughtful of them, this can be a source of much trouble. Additional scanning can be very resource consuming and result in delayed processing, freezes and other issues.
However every decent Antivirus software usually offers a real time protection for all downloads and thus this 'repeat' scanning is not of much use, rather it is...