Well if you think its Firefox scanning go to about:config and find browser.download.manager.scanWhenDone and toggle to false.
Could be Norton scanning before the file saves. Nod32 is having a problem like that in thew lastest build. Try disabling norton if turning off Firefox scan doesn't work.
Could be Norton scanning before the file saves. Nod32 is having a problem like that in thew lastest build. Try disabling norton if turning off Firefox scan doesn't work.