I just upgraded to NIS 2012 (v19.2.0.10) when my NIS 2011 had to be re-installed. But once NIS 2012 was installed, Thunderbird v8.0 could not log into the POP3 server. New emails were waiting download on the POP3 server but couldn't be downloaded. Outgoing email worked OK. I found that when I turned off "Email Protection," Thunderbird immediately was able to log into the POP3 server and download email. I confirmed that "Email Protection" was the culprit by turning it back on, which again blocked Thunderbird from logging in and downloading emails. So I have had to permanently turn off "Email Protection" in NIS. But NIS strongly wants to turn it back on. Hopefully, there will be a fix in NIS 2012 soon to work with Thunderbird so I can get Email Protection working again.
p.s. This PC is running Vista Home Premium SP2.