I read this on the blog of marcan:
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 issue was still there in Ubuntu 10.04 according to Steve. As I’m planning to install a test instance of Ubuntu 10.10 in the near future, I’ll keep this one in mind.