To begin, go to:
File --> New --> Get a New Mail Account
In the next text box, click on 'Skip this and use my existing email'
In the next text box, enter the following information:
Afterwards, click on 'Continue' located in the bottom right corner.
The next text box may try to automatically choose the incoming and outgoing settings. Since this setup is for Pobox Basic and Plus Accounts, you will need to change incoming server settings. You will also need to make a few changes to the SMTP configuration. To do this, click on 'Manual Config':
For your outgoing settings, enter:
For your incoming server, enter the following:
After entering all of the necessary information, click on 'Done' to finish setting up Thunderbird.