As of Thunderbird 3, the new Automatic Account Configuration Wizard can help you configure your connection to your email provider, but only if your provider is in Mozilla's database of Internet Service Providers or recognized by Thunderbird. While most of the large providers are included, many smaller ones are not yet in the account configuration database.
If your mail server supports both IMAP and POP, Thunderbird will use IMAP by default. POP is also supported but must be manually configured using the following instructions:
This tutorial will show you how to configure a new account for POP access (rather than IMAP). An existing IMAP account cannot be converted. Instructions for deleting it are at the end of this article.
If everything is ok (i.e. you can send and receive email and see your old emails; note that some email providers may not give you access to all of your emails or all your email folders; check with your mail provider) and you accidentally created an IMAP account, then delete the accidentally created IMAP account as follows: open Tools > Account Settings..., select the accidentally created IMAP account with the IMAP server setting, then select Account Actions > Remove Account from the drop-down list underneath the account list on the left.
For more information about account connection information, see Manual Account Configuration.