See https://support.mozilla.org/bm/questions/1249578

Deleted files as instructed and no change. My email folder is listed twice in the left column. One folder has a properties option and this is where my email resides upon receipt. The other has a Settings option that has much to offer but no email in it.

Running in safe mode removed the additional folders. Now to find the add-on I do not need.

I do not see any add-ons or a place to remove them.

I selected Help from the menu items and then “Restart with add-ons disabled”. A dialogue box poped up and I checked “Disable all add-ons” and also “Reset toolbars and controls”. Then I selected “Make Changes and Restart”. Additional account listings are gone. I still have “Local Folders” with “Inbox, Drafts, Templates, Sent, Archives, Junk, Trash and Outbox” and those folders are duplicates and not used. There is no way to collapse the folder or delete it.

Solved. I have 3 email accounts. Thunderbird created 3 separate areas for those accounts in the folder tree. The lower area has an "Inbox" and related folders plus all saved email folders where all the previous emails existed. The solution was to copy the folders in the lower section to the appropriate email account in the upper area. Then delete the files in the lower folders. This is a more practical way of operating. Then I simply collapse the lower area called Local Folders. I like this better than before when Thunderbird dumped all three accounts into one inbox.