Below are two example functions which could be used to to change the proxys via
a pac file

function FindProxyForURL(url, host) {
 return "SOCKS locallhost 1080";
}

function FindProxyForURL(url, host) {
 return "DIRECT";
}

Btw. if we support firefox dont forget to mention that the people have to set
network.proxy.socks_remote_dns..