Your account was POP but is now IMAP. You could have kept it POP by simply changing the authentication to OAuth2, not removing the account as Yahoo misguidedly recommended. Close TB, open the Mail/pop.mail.yahoo.com folder, copy the mbox files - large files with no extension, named after folders, e.g. Inbox, Sent - into Mail/Local Folders. When you restart TB, the folders will appear under Local Folders in the Folder Pane.

There may also be sbd folders in Mail/pop.mail.yahoo.com that contain mbox files.

Try to avoid copying messages from the old POP Inbox to the current IMAP Inbox, except in small numbers.

Thank you for the help sfhowes.

I did that and put the data into the 'local folders' place after I had it in the 'mail' folder but nothing happened in thunderbird when I had in the 'Mail' folder. Now having all the files pasted into the 'local folders' location the data does appear in thunderbird. But they are all totally mixed up with the other folder, having now two inboxes, etc. Something's off...

In my 'Mail' folder I see the pop.mail.yahoo.com folder from which I copied data out from. But there are other folders in the 'Mail' folder. Such as 'smart mailboxes', 'pop.yahoo.com', 'pop.yahoo-1.com'. Do I need to copy stuff out from these folders too to make it work properly in thunderbird?

You can always rename an mbox file, e.g. to Inbox OLD, after pasting it into Mail/Local Folders. If there are other mbox files in pop.yahoo.com or pop.yahoo-1.com (ignore smart mailboxes), they can be copied into Mail/Local Folders.

Thank you.

Yes, I understand. Still the data is now in a completely different messy form in my thunderbird account. Why did it not reinstate it all in the old form? Or more importantly, how do I get it into that form without having to laboriously rearranging/renaming/resorting everything again??

If you have a complex folder structure, the ImportExportTools NG add-on might be easier than the manual method.

Thank you very much for your continuing assistance!

How do I go about it? Do I FIRST delete all the data I just transferred into the 'local folders' manually? THEN after installing this add-on open it up in thunderbird and select its 'export directories' icon and select the pop.mail.yahoo.com folder to export as a MBOX file?

I would leave the manually imported folders as is, import the POP folders from the non-current profile folder, to Local Folders in the current profile, with the add-on, then delete the manual folders afterwards, in the TB interface, once the new imports are confirmed.

I used the add-on as you described but nothing ended up in the TB interface. When I use the browsing function in the add-on's 'export folders as a MBOX file' it ONLY gives me access to some saved pop.mail.yahoo.com folder that does NOT contain any of the old mfile folders (inbox messages, etc). This add-on seems totally useless.

How about reverting back to a POP account, and disabling/removing IMAP? I would not mind at all. That might make it works smoothly again. Does anyone know how to do that?

I reverted back to a POP account, using these guidelines at https://support.mozilla.org/en-US/kb/faq-changing-imap-pop

Then I fiddled again with the ImportExportToolsNG extension I had installed and managed to get practically all my folder and info back in place. Yippie. I do need to make some adjustments but at least I have it all back in TB.

"sfhowes"... THANK YOU SO MUCH again for your incredible help!!!!!! Your feedback was essential for me to solve my issue.

I reverted back to a POP account, using these guidelines at https://support.mozilla.org/en-US/kb/faq-changing-imap-pop Then I fiddled again with the ImportExportToolsNG extension I had installed and managed to get practically all my folder and info back in place. Yippie. I do need to make some adjustments but at least I have it all back in TB. "sfhowes"... THANK YOU SO MUCH again for your incredible help!!!!!! Your feedback was essential for me to solve my issue.

I marked this as the solution.