It should all be saved in the session.json file. Do you have a product like CCLeaner or WiseCleaner or even an Anti-virus product that offers a cleanup of files because they have a habit of cleaning up files such as temp or session etc files. You should not allow any of those types of Products anywhere near any Thunderbird file or folder. So if using those products then make all Thunderbird files and folders are exempt from any scanning or cleanin up.

Removal of Thunderbird and reinstalling is rarely required unless something went wrong with an update or installation. Removal of Thunderbird program has no effect on your Thunderbird profile. Most issues are in the profile not the program.

So, having made sure you do not have another program interfering, delete the session file and reset toolbars etc. In Thunderbird

• Help > Troubleshooting Information

Under 'Application Basics' - Profile Folder - click on 'Open Folder' A new window opens showing the contents of your 'profile name' folder.

Exit Thunderbird now - this is important.

The locate and delete the following files:

• panacea.dat
• session.json
• xulstore.json

Restart Thunderbird

This will reset everything to default. Then try setting up the columns as you would prefer and test to see if this worked ok.

Thanks for your suggestion, but it doesn't work. Previously, when I clean installed TB, I had removed all folders, including the profile. I installed the old 68 version and it works properly. The problem concerns 78 only.

No the problem concerns your profile and how it responded to conversion for use with 78 most probably, or even with an addon you may have installed. Have you tried Thunderbird safe mode? https://support.mozilla.org/en-US/kb/safe-mode-thunderbird

Yes, I have tried safe mode, but the result is the same.