The article about IMAP Synchronization explains the benefits of IMAP, which is the most modern and widely-used email protocol. (POP is an earlier protocol with less functionality.) If you want to convert your existing POP account to an IMAP account in Thunderbird, follow these steps.
While most mail providers support IMAP, a few still don't (including some well-known providers such as Hotmail). So first make sure that your mail provider supports IMAP.
If your provider does not support IMAP and you still want to use an IMAP account, you must choose a new mail account provider (for instance, those sponsored in Thunderbird support IMAP), create an account, and then move your messages from your old POP account to your new IMAP account, as explained below.
In order to avoid duplicating the checks for new messages, first go to your account settings by opening the ToolsEdit > Account Settings menu, then select Server settings and un-check both Check for new messages at startup and Check for new messages every XX minutes.
If you have messages or folders in your POP account that you would like access via your IMAP account, simply select them and move them to your IMAP account by dragging and dropping. Depending on the number of messages that you move, it may take several minutes to be uploaded. Be patient; you are nearly finished.
If you are converting your email account to IMAP from POP, you can delete the POP account when you are completely sure that you have moved all your important messages to the IMAP server (or into your Local Folders). You cannot easily undo this action, so make sure that you really don't need the messages stored in this account any more.
If you have created a new email account that uses IMAP, you cannot delete your old POP account without deleting your email address. Before deleting the POP account, you will probably want to inform people who send messages to your old account that they should switch to your new email address, and allow a period of time to pass before deleting the POP account to ensure that you don't miss any messages.