01 - Open a new tab in Firefox and paste in the address bar "about:config" and type Enter key (it's a configuration page that displays a lot of options... be careful)
02 - In the filter option, paste this "browser.download.manager.scanWhenDone" This way you get the only setting we want to change quicker.
03 - Double click to change the value to false, you will see after that the setting changed to black colour and says User set for the status.
04 - Restart Firefox and test by downloading a file to see if the virus scan starts.
This worked for me...