Try killing IPv6 system wide. What you have done only stops Firefox from making IPv6 requests. Any other IP application will cause timeouts, DNS lookup, or a mail program etc.
I had the problem on Mandriva 2007.1 and killed it by editing the start up scripts for eth0. Here is what I did to fix it. In the file /etc/sysconfig/network-scripts/ifcfg-eth0 I added these lines,
Code:
IPV6INIT=NO
IPV6TO4INIT=NO
You will have to figure out where your start up scripts on Kubuntu are, or possibly there is another way to stop IPv6, system wide on a Ubuntu flavor.
Hope this helps.