What is exactly the difference between these options? I have done a few tests but am not 100% sure. Currently I'm assuming extensions.update.enabled does only check for new versions of addons but it does not install them while extensions.update.autoUpdateDefault does only install addons if Firefox has already found a new version of them. In this case setting only extensions.update.autoUpdateDefault to false would disable the automatic installing of addons and setting only extensions.update.enabled to false would disable the automatic searching for new addons which also implicitly disables the automatic installing of addons. If my assumption is correct setting only extensions.update.enabled to false would be enough to disable any communication to an addon server.