This utility is a rebuild of an old program I made for my wife. Once she forgot her mail password, but it still was configured at her Outlook Express. So I've quickly made a program, which acts as POP3 server, allows user to authenticate and says there's no new mail. User name and password are then shown at program. Since Outlook Express doesn't check for server name change and keeps saved password when you modify account settings, so that time I've just set 127.0.0.1 there and got the password.
Thunderbird doesn't allow such tricks, it would reset the password and ask it again. To get it from Thunderbird (or any other mail client), open the file hosts at C:\Windows\System32\drivers\etc and add the following line:

127.0.0.1    mail.server.name

mail.server.name, certainly, should be changed to you mail server name, like pop3.mail.com or whatsoever.

This trick will assign the local computer address to your mail server and fool your mail client, redirecting connection to pseudo POP3 server.
BTW, if you have SSL connection, Thunderbird allows to change it's type to insecure without resetting password. At least it's so at 2.0.0.17.

Don't forget to remove that line after all, or you won't connect to your real mail server later.