Offline when Minimized — matroshka

Get the peace of mind that your open pages/tabs are not accessing the internet while their window is minimized. This saves some of the computer resources.
Firefox automatically reduces the javascript timer frequency when the tab is in the background, but does not prevent it from accessing the network. With this add-on you get the added benefit that a tab in a minimized window is unable to transfer any data over the net.

Notes:
* This add-on does not terminate connections that are already established when the window is minimized. It just prevents new ones.
* For a more radical solution, try the add-on "Auto Tab Discard" by Richard Neomy, which totally pauses the processing in background tabs.