Switch to the Light theme IF you're using the default theme. See if their display changes.

Same thing.

I copied a few of those "rogue" mails and one that is displayed correctly to another local folder. When I open the corresponding file with Notepad, this is what I get:

From - Wed Sep 09 09:53:46 2020 X-Mozilla-Status: 0001 X-Mozilla-Status2: 00000000 X-Mozilla-Keys:

From - Wed Sep 09 09:53:46 2020 X-Mozilla-Status: 1003 X-Mozilla-Status2: 00000000 X-Mozilla-Keys:

From - Wed Sep 09 09:53:46 2020 X-Mozilla-Status: 0013 X-Mozilla-Status2: 00000000 X-Mozilla-Keys:

From - Wed Sep 09 09:53:46 2020 X-Mozilla-Status: 0001 X-Mozilla-Status2: 00000000 X-Mozilla-Keys:

From - Wed Aug 07 10:01:29 2019 X-Mozilla-Status: 1001 X-Mozilla-Status2: 00000000 X-Mozilla-Keys: Return-Path: xxx Delivered-To: xxx Received: from xxx xxx

I replaced actual data with xxx for the only readable mail, the rest is meh, empty.

This sounds like you moved a load of headers from an imap folder into Local Folders account. When you 'Move' an email from an imap folder it gets deleted off the server because it no longer exists in any imap folder.

If you are talking about a gmail account, then it is possible only the label was deleted and the actual email is still lurking as an archive in the 'All Mail' folder. In which case, logon to webmail, and look in 'All Mail' folder and then you can drag back to Inbox to replace the label. In Thunderbird, click on 'Get Messages' and they should all reappear in Inbox.

Usually, I would advise that you make sure synchronisation is set up to download full copies. https://support.mozilla.org/en-US/kb/imap-synchronization#w_configuring-synchronization-and-disk-space-usage

Then go into offline mode via: Menu app icon > File > Offline >Download/Sync now Select checkbox for Mail Messages Select checkbox 'Work offline once download and /or synchronisation is complete'

Note the 'Select' button allows you to double check all the folders and subfolders you require are selected. Click on OK Download sync makes sure you are up to date.

Once in 'offline' mode, so that no synchronising goes on whilst you are getting copies of emails moved about, Highlight emails, right click and use 'COPY to' and select folder in Local Folders account. I advise 'Copy to' method because if something goes wrong then the originals are still in the imap folders. Check you can read various randomly selected emails to ensure all is ok. If you can read emails then you have fully downloaded copies of emails.

Go back into Online mode - in Status Bar at the bottom look for icon on far left and click on it. In versions 68* or previous it was a two black screen icon that chnaged to two blue screen icon when online. In version 78* onwards that icon looks like dot with radiating lines - tooltip when you hover will say eg: You are currently offline.

Once back online you can double check the Local Folders account if you feel it is necessary. Now delete the originals from the imap folder and then compact the imap folder. Server gets updated.

When you move an email, there is a load of cummunication going on between your computer and the server when it involves an imap folder. It is possible your Anti-Virus tried to scan the open files and messed up the contents or interrupted the download with trying to scan thousands of emails. Hence why I would advise going into offline mode to effectively disable that communication once you had full copies downloaded. It is also possible you never had full copies downloaded in the first place.

Have you ever created a backup of your Profile folders ?

Well, unfortunately, backup was made but not recently so those 5k mails are gone with the wind apparently. Synchronization is on and I did this process more than a few times before but now something went wrong, I don't know what. Nothing changed, antivirus is the same, everything... Well, I will use Copy option next time since I definitely learned a lesson now. Never imagined something like this could happen'.