From MozillaZine Knowledge Base
Starting with Firefox 3, the Windows security settings for downloading applications and other potentially unsafe files from the Internet are honored on files downloaded by the Download Manager. This allows, for instance, system administrators to block executable files from being downloaded by Firefox. This preference allows Firefox to ignore those OS-wide security checks.
Starting with Firefox 3.6, this preference is no longer used.
Possible values and their effects
Do not use Windowsâ€™
First checked in
Has an effect in