Ironically enough, whilst looking for your settings, I see your ISP has a page dedicated to helping Thuderbird users....
Set up your Yahoo Xtra email account in Thunderbird - Telecom New Zealand Help
IMAP settings are here
IMAP settings for Yahoo Xtra accounts - Telecom New Zealand Help
Although I would point out they do say IMAP is "unsupported".... so do at your own risk. No idea why they wouldn't support it. I reckon IMAP is by far the more superior option, I have not used POP3 since the dark days of the dial-up modem !