For somes reasons, firefox return IPv6 when you use the myIpAddress() cmd in a proxy PAC file.
This is a problem for me, because i don't use IPv6 in my office, and i can't apply correctly all directives.
So, you just need to go to in : - “about:config” in firefox
- search for the network.dns.disableIPv6 element
- set it to “true”
You can debug the myIpAddress() cmd, with an add-on “Web Developer”.
After install, open the debug window and you can see what IPaddress is returned.