Send packets across the Internet only when they come from user actions.
Firefox has a mind of its own. It sometimes connects to other computers across the Internet without asking you first. Not only is this a privacy issue, but it can also be awkward. For example, the browser might be installed on test equipment that is network-enabled only intermittently. If you are performing network diagnostics, that's another time when you don't want any unexpected chatter on the line. Finally, if you configuration-control all of your installed software, then you probably prefer that Firefox not upgrade itself automatically either. Here's how to stop all of that stuff.
Firefox periodically (daily) checks the Mozilla Update web site (http://update.mozilla.org) to see what's new. If there are critical patches, the home page displayed at startup is replaced with a warning page. If there are any patches at all, an icon appears on the menu bar. To turn off that functionality, set these preferences:
app.update.enabled /* default is false */ app.update.autoUpdateEnabled /* set to false. default = true */
The second preference stops Firefox from polling the web server to see if there's anything new to report to the user.
These two additional preferences do the same job as the previous preferences, but they control update checks for extensions, plug-ins, and themes rather than checks for the core Firefox product:
extensions.update.enabled /* default ...