Unfortunately the IPv6 issue is still at large in Karmic Koala and will effect just about every internet application.
The problem is caused by Ubuntu requesting everything by IPv6 first even if there are no IPv6 Interfaces configured and timing out each time when it doesn't get a response.
The workaround in firefox is to go to "about:config", just type it into the address bar and hit enter. Then change the value network.dns.disableIPv6 to TRUE.
The bug can be tracked here:
https://bugs.launchpad.net/ubuntu/+source/network-manager/+bug/417757Firefox Ubuntu 9.10 Karmic Koala DNS