This behaviours seems strange and unnecessary – Firefox doesntt have any buit-in antivirus, so it's not going to find anything if there is no antivirus software on your computer (even then Firefox will misleadingly display "Scanning for Viruses"). And if you have antivirus, real-time scanner will scan downloaded files anyway.
To disable this feature perform following:
- Open Firefox and type about:config in the address bar
- Find entry browser.download.manager.scanWhenDone and change its value to false