Is this a POP3 account or IMAP?

Have you checked your email provider's web mail interface to confirm that the 'missing' emails are there?

Do you still have access to the old computer, more specifically, to your old Thunderbird Profile?

I don't know how to tell if it is POP3 or IMAP. I think it is POP3. The emails are there on the old computer. Just not on the new one. I bought a laptop about a year ago and the same thing happened with it. This new computer has got the same starting email as the laptop has. It is like this new computer copied the laptop and not my old computer. Thank you for the help.

A quick lesson in POP3 vs. IMAP, if you don't know it already...

- POP3 means Thunderbird is set up to download email from the server and then delete the server copy, leaving the only copy of the message on your local computer.

- IMAP means Thunderbird will leave all email on the server and download a copy to your computer. Everything you do locally will be synchronized with the server copies.

When you first set up an email account in Thunderbird, you decide which of the two it will be. My point is, if your old computer was set up as POP3, the only copies of those messages may exist on the old computer and can no longer be downloaded from the server to the new computer. That could account for why you don't see as many messages as before.

Before I make any suggestions please provide some information:

What is your operating system? What versions of Thunderbird? Will you be comfortable copying folders of files from the old computer to the new via a thumb drive or across your home network?