Disable Automatic Virus Scanning in Firefox 3
Type about:config in the Firefox location bar and filter for the string browser.download.manager.scanWhenDone. The default value of that parameter is true which means that scans will be done whenever a file is downloaded. Setting it to false will disable the automatic virus scanning in Firefox 3.
Reference:
http://www.ghacks.net/2008/06/04/disable-automatic-virus-scanning-in-firefox-3/