Have you tried restarting Thunderbird in Troubleshoot Mode?

Try this: In Thunderbird

• Help > Troubleshoot Mode
• click on 'Restart'

Select checkboxes:

• Disable all addons
• Reset toolbars and controls
• click on 'Make changes and Restart'

Report on results.

Thanks, I executed the instruction, but I have still the same problem. Moreover, after executing the instruction the tab "e-mail" and all my e-mails disappeared, I clicked around and closed/restarted several times Thunderbird, and finally the tab & e-mails came back, but still impossible to have the "all folders view".

Try the following. Maybe a file got corrupted.

In Thunderbird

• Help > Troubleshooting Information
• Under 'Application Basics' half way - Profile Folders - click on 'Open Folder'

New window opens showing the contents of your profile name folder

• Exit Thunderbird now.

• Delete the following files:

1. folderCache.json
2. folderTree.json
3. session.json
4. virtualFolders.dat
5. xulstore.json

• Click on 'Mail' folder to see contents
• Delete this folder : 'smart mailboxes'

These files will get recreated when you restart Thunderbird. Start Thunderbird.

All mail to get checked and downloaded. Then check the 'View > Folders > All'

Thanks, I did all that, but the result is always the same. Is there a safe way to: - backup all the e-mails from Thunderbird (but no the settings) - wipe completely Thunderbird from the computer (also from registry) - make a clean installation of Thunderbird - restore all e-mails from backup Or reorganise folders manually, e.g. by drag-and drop?

I have found this: https://support.mozilla.org/en-US/kb/moving-thunderbird-data-to-a-new-computer, but it looks like it transfers everything including settings, so that would not fix the problem, true?