-Your Name: Display name you wish email recipients to see
-Email address: Your Pobox Address
-Password: Your Pobox Password
Be sure to check "Remember password" Then click Continue.
SSL/TLS (This is important! Using STARTTLS will cause errors and unusual disconnects.)
Then make sure the Incoming line has been switched from "POP" to "IMAP", and click "Manual Setup".
Server Type: IMAP Mail Server
Server Name: mail.pobox.com
User Name: Your Pobox Address
Connection security: SSL/TLS (this is important! Using STARTTLS will cause errors and unusual disconnects.)
(NOTE: Thunderbird 3 seem sluggish to you? You aren't the only one. Click that link to go to Mozilla's community-powered support page for this issue. There, you can compare your program's performance with other Thunderbird users' slowness reports, and try a few possible fixes at your own risk.)
(You may also want to refer to this page here for another configuration change. Thanks to Wayno for the details!)