Is my Thunderbird account IMAP or POP3?

click 'account settings' then click the server settings' pane for the account. The top line will specify IMAP or POP. Ideally, it is IMAP if you plan to share with phone,

Yes, thank you. My Jitterbug phone reeds IMAP, but I don't know how to convert from POP3.

So, the real question is 'how to convert to IMAP.' You don't. You just simply create a new account in thunderbird, specifying IMAP instead of POP. Most vendors use different servers and, most likely, thunderbird already knows them. SUMMARY: you cannot convert an account, you must create a new one. And they can coexist.

Thank you very much.

You're most welcome.

Thanks again. One last important question - If I do shut down both of my POP3 email accounts and reopen them as IMAP, can I still keep my present addresses? Or will I need to alter them?

Your addressbook is a separate entity, not tied to accounts, so you're safe there.:) I suggest you keep the POP account initially, as both can be there. My concern for you is that messages already downloaded will be lost if you delete the POP account. if there are messages in the POP account you want to retain, I encourage copying them to Local Folders before shutting down the POP account

So I'll keep them both up. But then must I shut down the POP account at some point?

You do not need to shut the POP down at all. It just depends on what you want. If you want everything visible in IMAP, then the POP account needs to be set to leave all messages on server, and that will cause your POP message folder to keep increasing. After using the two for a while, I think you will be able to decide what you need for the future.