Ever since download managers have become popular, the auto shutdown feature has become standard with these managers. Even peer to peer software have featured this option. While Firefox integrated a relatively better download manager (compared to IE), it doesn’t carry that feature. But at least you can install an extension that lets you do so.

Auto Shutdown adds an icon to your status bar and the download manager window. Clicking the shutdown icon will set Firefox to shut down the computer when all the downloads have either finished or failed. You don’t have to wait around until that download finishes anymore.

Download Auto Shutdown - InBasic here.