Since you're using IMAP, they will in all probability disappear unless you do something about it. If you want to keep them, you need to make copies outside of the Yahoo account.

Thunderbird provides the Local Folders account for just this purpose. Move your messages there before it's too late. Since you're going to be getting much of the material from yahoo's servers, i.e. via your internet connection, try to do this as a large number of small transfers.

I understand "headers only" to be specific to POP-connected accounts.

If you are not going to read the answers to your question, why post it again?

Here is your original question.

https://support.mozilla.org/en-US/questions/1013151

Thanks, Airmail. I'm new at this forum stuff and could not find my original post so I thought it had gone into cyberspace. Thanks for taking the time to answer.

You can click on your name beside one of your posts and be taken to your profile/posting list. I would suggest doing this and then bookmarking that page so you can find your threads easily.