At first launch of firefox, to a site behind our proxy, the request for credentials still pops up. It only happens once, and then doesn't come back, but we don't give proxy access to everyone, and don't want it popping up at all unless they need it. I've removed as many auto update preferences as I could find, but it still happens. We connect to the proxy using an autoconfig script. We are working with thin clients so I don't have the ability to just upgrade software. But I can change the preferences at will.