Watching and waiting for huge files to download is frustrating enough but knowing you have to wait for them to finish just so you can shutdown your system makes it worse. You can always let them complete and let your system go to standby but if you prefer to shutdown your system at the end of the day and find yourself babysitting your downloads just to do it, try Auto Shutdown NG. It’s a Firefox extension that automatically shuts down your system when all downloads have completed.
The extension adds a power off icon to the Firefox download manager. To activate the extension, set the shutdown command (choose your system) from the extension’s options. When download starts, the shutdown button is grayed out indicating the extension is inactive. Click the button and it will turn red. Once all queued downloads are complete, a dialog box opens indicating time remaining until system shutdown. When time runs out, the extension will initiate system shutdown. You can cancel the shutdown by clicking Cancel or force it early by clicking Shutdown.
When shutting down, the extension exits all programs that are currently active.