Gecko.GeckoPreferences.User["network.proxy.type"] = 1;
Gecko.GeckoPreferences.User["network.proxy.http"] = address;
Gecko.GeckoPreferences.User["network.proxy.http_port"] = portNumber;
Gecko.GeckoPreferences.User["network.proxy.login"] = Username;
Gecko.GeckoPreferences.User["network.proxy.password"] = Password;

address  = 127.0.0.1
port     = 8008
username = ""
password = ""