Huh, so they have Firefox locked down? Or you're trying to use Firefox to get out from under the group policy-dictated proxy auto-configuration Javscript file for their network?

I used to manage this sort of thing for my company. If I recall, the auto-conf script determined which sites were allowed to pass through the firewall, and which were required to pass through the network proxy.

Larger organizations, and particularly schools that I used to manage, would use the Microsoft Active Directory server's group policy (applied to each user organization-wide) to force Internet Explorer to use a specific URL for their Javascript proxy auto-config file (PAC file). When using IE, the browser would the access the Javscript at that location, and behave according to the rules set forth in the PAC file.

Clever students would figure this out, and then try to download Firefox so that they were out from under IE. Given that we were jerks, and wanted to teach the students the lesson that creativity doesn't pay, we actively denied any HTTP traffic that was not explicitly proxied through our chosen servers. This means that even if they figured out a way around the proxy or their group policy-enforced PAC file configuration, the network itself would prevent them from accessing inappropriate content.

If your network admins are worth their salt, they should have locked this down as well.

The most common way to get around this is a method called "tunneling," which takes traffic of one type and masquerades it as traffic of another type, with some third party entity arbitrating. This usually involves an encrypted SSH tunnel, which is also pretty easy to block if your network admins are paying attention.

However, if you just want to configure Firefox to use your school's PAC file, these look like decent instructions (techlib.barracuda.com). Set the Automatic Proxy Configuratoin URL to the same as what it is in IE, and you should be set. Just remember to unset it when you go off-network.