Comment 13 for bug 417757

Just updated to karmic and experienced the same problem (1-3 second delays on dns lookup).
Switching off IPv6 dns support in firefox "fixes" the problem.

Do do that open up Mozilla or Firefox and type in 'about:config' in the address bar
Scroll down to "network.dns.disableIPv6", it's defaulted to a value of false, change it to true.

hope that helps.