[eitch100]

Diese Option gibt es anscheinend seit Firefox 31 nicht mehr, sondern nur noch in Seamonkey...

Disable virus scanning in Firefox preferences - Windows
In Firefox 3.0 and above, up until recently, Firefox launched your installed anti-virus program to scan files when the download completes. [30] In some cases, this causes a substantial delay or hang while the file is scanned. [31] [32] In other cases, the file is not saved after the scan completes. [33] [34] [35] You can disable this feature by setting the preference browser.download.manager.scanWhenDone to false in about:config. [36] Note: If the Download Manager appears to be stuck scanning for viruses, try closing and re-opening the Downloads window instead of disabling the preference. [37] [38]
The browser.download.manager.scanWhenDone preference is no longer available in current Firefox versions. It's still included in Firefox 24 but was removed since at least Firefox 31 (the preference is still present in SeaMonkey 2.26.1 which is based on Firefox 29). Firefox 31 includes a new feature that scans downloads and blocks those that may be malicious. To turn off this new feature, modify browser.safebrowsing.appRepURL in about:config and replace the value with an empty string. This disables application reputation checks but leaves other Safe Browsing malware protection intact. [39] [40]

Ich kann da leider nicht weiter helfen... übrigens, der Virencheck wird nicht von Firefox ausgeführt, sondern von deiner Antivirensoftware.