Does it run any better in Safe Mode?

https://support.mozilla.org/en-US/kb/safe-mode

If so, you may have an add-on causing trouble. Disable them all and re-enable one by one (though, thinking again, I'm not sure how you'd disable them in a way that would let you re-enable them one by one.)

Thank you for the suggestion. I tried to run in safe mode, with the combinations of "disable add-ons" and "reset toolbars - controls" options. Each time the system rebooted as soon as I run, except for the time when these options were not checked. Thunderbird run for a few moments and as the new messages arrived I received errors like these: "cannot write message to folder xyz. No space left or not enough permissions..." (I have some filters for message groups, people etc. Some of them worked fine but other failed with errors similar to above.) And then system rebooted.

Afraid of loosing all the mail archive, I backed-up everything with mozbackup. I will try a clean installation and try to restore.

I don't think a clean install will help.

Try Windows safe mode with Networking. and see if similar issues occur. If they do, run a disk scan for disk errors.

A rash of space ans security errors sounds like something else is trying to access the same files at the same time. So my first guess is anti virus trying to scan mail folder while Thunderbird is trying to write to them and a mess ensuing. Thunderbird has a new version number so everything it knew about Thunderbird was for a different program.

Disk errors however can never be excluded, even on relatively new hardware. (most manufacturers tolerate a certain level of bad sectors in their new products they are so common.)

As an addendum... You appear to be using Chrome as a browser... could you try the current version of Firefox and see if it also causes a reboot.

Restarting is a kernel mode event. Normal programs don't have access to the CPU at that level Anti virus do as does all drivers including the display driver.

Firefox (and Thunderbird by default) have had vidoe hardware acceleration turned on by default in the last version. If Firefox also fails, that may be the problem.

Thank you for the suggestions.

No log entries in Comodo about Thunderbird profiles. Firefox works fine (although mine is not the latest. It used to be my best browser but I'm using Sleipnir - yes, a Chrome based one - for some time because it is really good for eyes and I am quite an old man)

But HD tune disk status showed me some S.M.A.R.T. entries; interface CRC and airflow temp errors..! I'm going to make a surface scan overnight and probably have to replace the disk but at the moment I'll make another copy of the profiles to another disk and try my chances with a clean install. (It seems that it is only a write error. Mozbackup could read the profiles.)

A clean install and restore from Mozbackup and a couple of restarts solved the problem. I'm going to replace the disk asap.