If you tend to leave your computer running overnight to let some Firefox download complete, now you can do so without the guilt of all the wasted energy once downloads complete and you’re well into R.E.M.
Auto Shutdown is a Firefox extension developed by InBasic that monitors your active downloads and will shutdown your computer when they are done.
Actually, Auto Shutdown is much more flexible than that. It can execute any command your enter when downloads are completed, shutting your computer down is just the default (which by the way, I think should be stand by).
It also supports the popular DownThemAll! extension which provides more powerful download management options.
You can get Auto Shutdown from Mozilla Add-ons.