From MozillaZine Knowledge Base
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 preferenceâ€”controlling whether the virus scan is automatically triggeredâ€”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.
First checked in
Has an effect in