You don't mention the location of the old files. Where are they located specifically? How many active accounts do you have in Thunderbird and where do they store their messages data? You can find that out in Account Settings. The parent folder containing the old files should be named after an incoming mail server. Do you recognize that server name or not? If you do, is it a server for a defunct account that you removed from Thunderbird a long time ago? If it's the server for an active account, maybe you reconfigured your accounts to store messages in the Local Folders account instead of each account having its own separate mailbox, as is the case when using a Global Inbox. It's only you who can tell us how you have your accounts setup in Thunderbird and whether those old files are no longer needed. If they were used by a POP account, it's possible they contain important messages that were deleted from the server after they were downloaded, and they are not in any other folder in Thunderbird.

This has nothing to do with Thunderbird version upgrades. It is a result of changes in account configurations that you may have performed long ago and have since forgotten, or had someone else do and you're really not sure how it went.

Thanks for the reply. To clarify, the old files are in: C:\Users\..\AppData\Roaming\Thunderbird\Profiles\5yx36fcc.default and include files and folders with dates since 2014 (e.g. places.sqlite with date 2/16/2022). The new files are in: C:\Users\..\AppData\Roaming\Thunderbird\Profiles\5yx36fcc.default\Mail\Local Folders In the "Mail" folder, along with "Local Folders", are folders for other email accounts and only one file, "News & Blogs.msf". You mention a "Global Inbox". I think that is what I used to have, but something changed (an account?) and I was not able to get Thunderbird to continue with that configuration. At that time, I did not look into what happened but I think I did not loose any email... Is there a way to have Thunderbird connect to the old Inbox file so I can look at it with Thunderbird? I don't see any option in File/Open to connect to another Inbox file.

Posting a screenshot of the mail folder and the local folders, showing files in each, may be useful.

Are you saying the old files (mailbox, msf and .sbd folders) are in the ROOT of your profile folder AppData\Roaming\Thunderbird\Profiles\5yx36fcc.default and NOT inside the Mail or ImapMail subfolders like those of your other accounts?

Yes, that is correct. Attached is a screen shot of the "Local Folders" folder. The root folder has 225 files and folders so I didn't attach a screen shot from any of it (but I can if needed). JimC

What you posted is a screenshot of the Mail folder, not the Local Folders. If that is where the majority of files are, then yes, posting a screenshot of that would help, possibly sort descending by folder size to get the major ones in screenshot?

Instead of a screenshot, here is a listing (from cygwin) of the files and "folders", sorted by decreasing size. I don't know how to change the font to fixed size to make the sizes more obvious. If this isn't useful, I will do a screenshot. How does this help with the issue about the files and folders in the "...default" folder? JimC

/cygdrive/c/Users/jcrapuchettes/AppData/Roaming/Thunderbird/Profiles/5yx36fcc.default/Mail/Local Folders $ ll|cut -c 34-|sort -nr 3947355331 Feb 17 16:19 Inbox

132567586 Dec 14 14:08 FindFish
 91344610 Dec 10 17:39 Amazon
 37076331 Feb 16 12:37 Drafts
 34126180 Sep 27 21:58 AHA1a58a4fd
 24506375 Oct 28 17:30 Gritman
 17059477 Feb 17 16:24 Inbox.msf
 10733672 Feb 10  2021 WSE Alumni
  8827455 Feb 10  2021 JBOD
  7923163 Aug  3  2016 Job search
  7420643 Dec 14 13:32 Microsoft
  3371987 Sep 23  2010 Don Gaubatz
  1915264 May  7  2017 NSA
  1907660 Dec 15 10:30 FindFish.msf
  1888779 May 31  2020 First Step Internet
  1398417 Dec 11 13:14 Amazon.msf
  1321704 Sep 27 21:58 AHA1a58a4fd.msf
  1098736 Apr  5  2018 7a9d0ade
  1064112 Feb 17 14:26 Junk
   793831 Feb 10  2021 JBOD.msf
   535179 Nov  2 16:31 Gritman.msf
   474665 Feb  1  2012 Church
   334709 Feb 16 21:21 Trash
   217149 Dec 14 13:32 Microsoft.msf
   196821 Aug  3  2016 Job search.msf
   124044 Feb 10  2021 WSE Alumni.msf
    66721 Jan  4  2010 DBIOfc01cea5
    66610 Dec 13  2016 RT-11
    59947 Jul 27  2010 3281d563
    53893 Sep 27 21:59 Don Gaubatz.msf
    36643 Sep 27 21:58 7a9d0ade.msf
    31742 Feb 10  2021 First Step Internet.msf
    23200 Feb 17 16:21 Junk.msf
    17300 Sep 27 21:59 Church.msf
    13372 Dec 14 13:28 RT-11.msf
    12351 Dec 14 13:28 3281d563.msf
    11185 Sep 27 21:59 DBIOfc01cea5.msf
     8555 Dec 14 13:31 NSA.msf
     8411 Feb 16 21:26 Trash.msf
     4372 Sep 27 21:58 Canon Press.msf
     3657 Feb 17 06:53 Drafts.msf
     3534 May 21  2016 Personal.msf
     3506 Oct  7 22:24 Unsent Messages.msf
     3254 Dec 14 14:41 Archives.msf
     3201 Dec 14 14:41 Sent.msf
     3101 Feb 10  2021 Canon Press
     2900 Sep 27 21:59 Consulting.msf
     2761 Sep 27 21:59 ESSac24d179.msf
     1710 Jan  6  2006 Personal
     1200 Feb  2  2010 Templates.msf
     1098 Jan  2  2010 Rachel's e-mail.msf
      198 Dec 17  2019 filterlog.html
       27 Jan 16  2016 msgFilterRules.dat
        0 Nov 17  2019 AHA1a58a4fd.sbd
        0 Nov 10  2019 Inbox.sbd
        0 Nov 10  2019 Consulting.sbd
        0 Mar 29  2007 Consulting
        0 Mar  5  2018 Sent
        0 Jun 19  2017 Unsent Messages
        0 Jan  2  2010 ESSac24d179
        0 Dec 14 14:43 Personal.sbd
        0 Dec 14 14:43 FindFish.sbd
        0 Dec 14 14:43 Church.sbd
        0 Dec 14 14:43 Canon Press.sbd
        0 Dec 14 14:43 7a9d0ade.sbd
        0 Dec 14 14:42 Job search.sbd
        0 Aug 30  2020 Amazon.sbd
        0 Apr 29  2021 Trash.sbd
        0 Apr 19  2011 Archives

First, to view contents, I suggest a simple freeware program, EDXOR. It is like notepad on steroids. From my experience, sbd files are no longer used, but worth a check. Also, you may wish to confirm the actual folders in use for your account by right-clicking those folders from left pane and clicking Properties.

I don't see how a directory listing of Local Folders is helpful since the old files in question are in the root of your profile folder. No account should be storing files in the root of your profile folder. IMAP accounts store files in the ImapMail folder, while POP accounts store them in the Mail folder, as shown in your previous screenshot. They are, therefore, out of place in the root. Create a new folder in a convenient location such as the desktop and give it an appropriate name, e.g. OLD. Select ALL those old mailbox and msf files, and the *.sbd folders and move them to the OLD folder. This should remove them from the root of the profile folder. It will also confirm if they are still in use by some account or not, because if they are, you will not be able to move them while Thunderbird is running.

You don't need any other program except Thunderbird, to view the contents of the old mailbox files. In Thunderbird, create a new folder under the Local Folders account and give it an appropriate name, e.g. 2014OLD. Create a subfolder under 2014OLD and call it Temp. Next, go to your profile folder\Mail\Local Folders. You will find a "2014OLD.sbd" folder. Copy (or Move) the contents of the OLD folder into the "2014OLD.sbd" folder then restart Thunderbird. You will now be able to access the old messages in Thunderbird, under the Local Folders account > 2014OLD.

Thank you both for your help so far. I am working on doing as suggested by Stans but ran into problem. When moving files from the "old" location (the profile root), I did not exclude the "ImapMail" folder. In the process, I ended with 2 different INBOX files, a short one which moved, and a big one which did not move. I moved everything back except the short file; the short one I renamed and then moved back with the big one. I think I read somewhere that if I were to delete/rename the INBOX.msf file with TB closed and then restart TB, it will re-create the .msf file. If that will work, I will do that to be sure that the file is correct. and then will follow Stans directions to create a new mail folder for the short file, move it with TB closed and then restart TB to create a proper .msf file for it. Does this all make sense? JimC

If the short inbox file is an msf file, don't worry about it. You might as well just delete all the msf files that are in the root of the profile folder. They are of no consequence because they don't contain any data that is worth saving. Thunderbird will recreate them when and where it needs them, so it's safe to omit or remove them from the process I described. They are merely index files for the mailbox files (the ones without extensions, such as the huge INBOX file).