What you should do, rather than disable updates (a horrible administration decision which leaves your machines and users vulnerable to known security exploits and bugs) you should either:

1. Fix your software

1. Use Firefox ESR, which is meant for large organizations. http://www.mozilla.org/en-US/firefox/organizations/

You can use a mozilla.cfg file in the Firefox program folder to lock prefs or specify new (default) values.

Place a file local-settings.js in the defaults\pref folder where you also find the file channel-prefs.js to specify using mozilla.cfg.

pref("general.config.filename", "mozilla.cfg");
pref("general.config.obscure_value", 0); // use this to disable the byte-shift

See:

• http://kb.mozillazine.org/Locking_preferences

You can use these functions in mozilla.cfg:

defaultPref();  // set new default value
pref();         // set pref, but allow changes in current session
lockPref();     // lock pref, disallow changes

You can consider to lock these prefs:

• lockPref("app.update.enabled", false);
• lockPref("app.update.auto", false);

Thanks so much for this answer. I was about to jump out of the Window because of this &§%$ AutoUpdate function...

Just let me add a little comment...

This:

• lockPref("app.update.enabled", false);
• lockPref("app.update.auto", false);

sets the FFX to "auto-check" and manual installation.

This (lines just switched):

• lockPref("app.update.auto", false);
• lockPref("app.update.enabled", false);

fully disables the auto-update and even greys out the Manual update function in the "about" window.