Note: This preference is no longer available since at least Firefox 31.  
In Firefox 3, if a Windows user has an antivirus program installed, it is launched to scan files when they finish downloading. During testing of the feature, concerns about delays and double-scanning files surfaced. As a result, this preference—controlling whether the virus scan is automatically triggered—was created.
Starting in Firefox 3.7 (3.7a3pre nightly builds since 2010-03-06) this preference also controls whether or not the Windows security policy checks are applied for downloading and launching executable files.  
Possible values and their effects
Scan files for viruses when they finish downloading. Starting in Firefox 3.7, also apply Windows security policy checks. (Default)
Do not scan downloads for viruses. Starting in Firefox 3.7, also bypass Windows security policy checks.
- In Firefox 3.6 and below (at time of writing) this preference is only used for scanning completed downloads and only has an effect if you have antivirus software installed and are running Windows.
- Starting in Firefox 3.7 (3.7a3pre nightly builds since 2010-03-06) this preference, when set to false, also bypasses Windows security policy checks. Note: Starting in Firefox 3.6, browser.download.manager.skipWinSecurityPolicyChecks is no longer used.
First checked in
Has an effect in
- Firefox 3.0b3 and later (nightly builds since 2008-01-30)
- SeaMonkey 2.0b1 and later (nightly builds since 2009-05-30)