Of course you can. If you currently have it setup as an IMAP account in Thunderbird, you can copy your messages to folders under the Local Folders account. Those will remain on your computer even after you disconnect from the IMAP Yahoo account (e.g. after deleting your account). If it's setup as a POP account, you don't have to do anything else except download them from the Yahoo server IF Thunderbird hasn't already downloaded them all during your day to day usage. No need to add another account. To read the copies, you simply read them in Thunderbird the same way you've been doing it, for as long as that Thunderbird data is accessible. Please note that you are fully and solely responsible for keeping your data safe. Don't wait until you don't have access to the only remaining copy of your precious data.