Check for a damaged prefs.js file:

https://support.mozilla.org/en-US/questions/1324156

Thank you for providing me a reply. I had already checked the file prefs.js and I have already tried to use the 2 older versions of this file, namely prefs-1.js and prefs-2.js. Unfortunately, the problem is the same. It seems that prefs.js is not the only problem or there is another problem somewhere else.

First, did TB create a new profile, or has it just lost contact with the existing profile? If the address book is intact, it s probably the latter. If a new profile was created, see if you can launch the previous one from Help/More Troubleshooting, about:profiles. If there's only one profile, consider creating a new one (from about:profiles) and add the accounts. Local mail, address books etc. can be transferred from the old profile to the new if necessary.

TB seems to have lost contact with the existing profile. And, yes, the address book is intact.

I went to the "about.profile" and only 1 profile was present and active.

Then ... what should I do ? Create a new profile by adding a 1st account ? If I do so, how can I transfer (move) all the other data (local mail, address books etc.) from the old profile ?

Local mail is stored in mbox files in Mail/<popservername>, Mail/Local Folders, and sbd subdirectories, in the profile. The mbox files are the large files with no extension, named after folders, e.g. Inbox. Copy the mbox files into the Mail/Local Folders location of the new profile. Copy abook.sqlite (Personal Address Book) and history.sqlite (Collected Addresses) from the old to the new profile.

Let me try ... but before ... Is it OK to just copy the complete Local Folders from the old profile to the new one ?

By the way ... thank you for your support !

Let me try ... but before ... Is it OK to just copy the complete Local Folders from the old profile to the new one ? By the way ... thank you for your support !

You can try copying the entire Local Folders, and if it fails, copy the mbox files.

I am actually trying to make the complete copy and the Local Folder tree (under TB) is now visible which is already a big step ahead. Now - when looking inside one of these boxes - it seems that it takes time to read its content. And ... it seems to work ! THANK YOU !

I have now 2 questions : - in one of my account, I had a huge list of e-mails (inbox) --> when I create again the account, how can I fetch this huge list ? Where is it ? Same for the outbox (of the account) ? - there are also other quite big files I have right under the OLD profile --> are there other files which I could copy ? I mean I had a quite powerful setting !

Is the 'huge list' for a POP or IMAP account? If it's POP, add the account and copy mail from the Mail/<popservername> folder that was added to Local Folders. If it's IMAP, the folders will download from the server when connected.

You can copy any mbox file from the old profile to Mail/Local Folders in the new one. Ignore the like-named .msf files, as they are recreated when TB restarts.

THANK YOU AGAIN ! I can see that you may be at the end of your day (as I am in Japan, it means for me the end of my morning). I will then try to find the right folder and do the right copy (it is a POP account). And I will let you know probably tomorrow morning (my time). However here is probably a basic question : what is/are the main files I should SYSTEMATICALLY backup ?

The simplest and most complete backup is to copy the profile folder or the containing Thunderbird folder, which contains all data and settings. Its also worthwhile to export address books to LDIF files and local calendars to ics files before the major, annual updates. Backing up is easy, and restoring from backups is only a bit more involved once you understand the structure of the profile folder.

http://kb.mozillazine.org/Files_and_folders_in_the_profile_-_Thunderbird

Because I still "dream" to be able to recover my corrupted profile, I need to ask you another question ... about this prefs.js file. The 2 other prefs files I had were : prefs-1.js and prefs-2.js. One was dated in February of this year and the other one in December of last year ! How comes that it didn't work when I tried to use these files ? More precisely : did I miss something here (like touching somewhere else) ?

No idea why the old ones didn't work. Perhaps compare them with the one in the new profile.

NOW I GOT BACK MY INBOX of ... 2,821 e-mails !!!!! THANK YOU !

I will stop - to disturb you too much (at least for today) - with this last question : Background : in my primary account (POP), the "outbox" was listed under the Local Folder but now it is listed right under the account itself ! Questions : it is possible to change it ? If yes, how ?

The Outbox is under Local Folders for POP and IMAP accounts, as you can confirm by using Send Later. No idea why it's under the account, but the location doesn't matter as long as it works properly.

THANK YOU AGAIN ! As I said earlier I will now work on re-establishing my settings and most (if not all) accounts I have. It will take me a few days of work (not full time of course). I like to come back to you (probably next week and not tomorrow as I has stated above) should I need again your expertise. And if I am OK - by my own - I will still let you know AND, at the same time, close this Q&A. I will be back !

To sfhowes ... THANK YOU VERY MUCH FOR YOUR KIND ANSWERS you provided to me last week.

All is now working normally. In the end, I found out some other (big) problems with my computer and did decide to buy a new one ! Exit Windows 7 (which cannot boot normally) and I am not using Windows 10.

But, and this was critical, I could re-install successfully Thunderbird, my settings and all the data !

Case closed.