So there I was, very happy with a clean install of Ubuntu, working nicely after the unresolved, random lockups of Linux Mint. The main fly in the ointment was the hideous amount of time waiting for DNS lookups in Firefox, as evidenced by the "Looking up www.domain.com.." in the web browser's task bar.
I noted a few suggestions about editing /etc/resolv.conf and entering the ISP's DNS server(s) rather than perhaps a local one, but this wasn't the problem.
Turns out 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. That's a bit crap really considering, but relatively easy to fix.
Open Firefox and in the URL field, type:
Press any 'Be careful' warnings and seek out the value network.dns.disableIPv6. Double click on this to change the end column to TRUE and close the browser page..
DNS lookups should now default to using IPv4 and be notably speedy. Note that for users with Linux systems behind a gateway, the first tip about changing the /etc/resolv.conf to the ISPs DNS addresses, rather than the IP of the gateway, can sometimes prove beneficial if there are lookup problems.