If you add a proxy (manual) to firefox, you can also put in exemption for which domains the proxy should not be used. And from the examples that are shown, you might think it is also possible to add ip-address-range like 192.168.1.0/24. But this does not really work. Even when your servers are in the network 192.168.1.0/24, the proxy still we be used. It is only working if you add your domain e.g. .mydomain.local.