Moving thunderbird mail from one computer to another.

In the last year, I've moved from one computer to another computer to still another compute and I need to move the mail I've collected from the first two computers to the last computer so I can access it using Thunderbird. I found a tab that allows me to import mail from Eudora and Outlook, but nothing for Thunderbird. I've also searched Thunderbird support but I couldn't find anything. There is information I need stored in these old files. Can you help???

Thank you!
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