On Ubuntu 11.04 64-bit, the following proxy configuration file doesn't work because the myIpAddress() returns always 127.0.0.1 instead of the "real" IP address. (Got this info via alert(myIpAddress()), which writes to the error console in Firefox.)

Code:
function FindProxyForURL(url, host) 
{
if (myIpAddress() == "192.168.1.6") return "DIRECT";
else return "PROXY 172.16.1.4:8080";
}
The answer on Wikipedia of removing entries from the /etc/hosts file didn't do it.

Any further clues what could get the myIpAddress() return the IP address (and not the loopback device's address)?