Mozilla has decided to disable the checkbox to stop checking for updates. About:config options don't disable the prompt to update either. Those would be the following:
// Disable updater lockPref("app.update.enabled", false); // make absolutely sure it is really off lockPref("app.update.auto", false); lockPref("app.update.mode", 0); lockPref("app.update.service.enabled", false);
No longer works entirely.
It appears that I can change this setting and it will work:
You can also delete the updater.app that resides within the Firefox.app.
However, what are you guys doing? I leaning towards redirecting the app.update.url.
Edit: Fixed a type and have confirmed that the app.update.url change does resolve the problem. Going with that.