Firefox doesn't do the "AV scan", it hands the scan off to the "registered" anti-virus application that you have installed. The "scan" should be done so fast that you shouldn't see more than a flash instance from Firefox of a scan being performed on the downloaded file, even if it is a large file like a 360Mb ISO image. If you saw (or see) the download scan dialog run for like 4 to 5 seconds, or more, '''a scan isn't actually being performed''' - Firefox is giving you a misleading indication that a scan is being performed, when it isn't. ''It's like Firefox is blowing smoke up your arse, with a false indication. '' I filed a Bug about this issue almost two years ago and it remains UNCONFIRMED. I only discovered it because I was doing am initial load of software on a PC that I had just built for myself, the first brand new one in over ten years, at the time. I was using a newly installed Firefox to download the Anti-Virus application I was going to install, and Firefox showed the download was being scanned, when I didn't even have an AV program installed on Windows 2000!<br /> https://bugzilla.mozilla.org/show_bug.cgi?id=480855 Three similar Bugs that others have filed. <br /> https://bugzilla.mozilla.org/show_bug.cgi?id=461989 <br /> https://bugzilla.mozilla.org/show_bug.cgi?id=501645 <br /> https://bugzilla.mozilla.org/show_bug.cgi?id=530307 I can't even find a list of the exact anti-virus applications that have the proper "hooks" in Windows that Firefox uses to "connect" the file being downloaded with the AV application - from either Microsoft or from Mozilla. IMO, that feature is broken at least, and at the worst it gives an invaild indication that a scan is being performed when it isn't because the user either doesn't even have an AV application installed, or has an AV program that isn't compatible with this feature - I think Firefox should tell the user in both those situations.