There's a plug in for Firefox that will handle the Windows Authentication for you but I can't remember the name of it ATM.

But that's not your question. There's two ways that it won't try to automatically authenticate and it depends on how you want it to work. If you don't want it to automatically ANYONE ever then just comment out the <add name="Authentication..... /> in the <httpModules> section of your web.config. That will stop the provider from loading but users can still manually logon. If you only want people outside your intranet from automatically logging on then use the IP Config at the bottom of the settings to set what IP ranges you want to attempt to automatically login.

If it's only certain groups within your intranet (I don't have the previous posts here when I reply but I think you are in an educational setting) then you're going to want to keep the "Don't automatically create accounts" checked AND use the plugin for Firefox. That option is there for schools/large business where they don't want accounts being created for outside the department. However, it still has to go through the process of "logging" the user in so that it can check to see if the user already has an account (if they do then the login is completed, if not it jumps out of the procedure).