Firefox:Disable Virus Scan on Download
When Firefox finishes downloading items, by default it does a virus scan. This can can cause FF to hang for a bit as well as prevent you from accessing the file until the scan is done. If you have antivirus software installed on your system, it will already prevent any viral activity via it’s real-time scan.
You can disable this feature by typing about:config in the address bar, and then filtering on browser.download.manager.scanWhenDone. Double click this line to set the value to false.