If it works in safe mode, it means there is either an incompatible add-on or theme, or a broken layout. There are other possibilities, but those are the most common. Check your add-ons, or repair the layout by launching in safe mode and checking 'Reset toolbars and controls', then 'Make changes and restart'.

I performed the 'Reset toolbars and controls', then 'Make changes and restart' method but nothing changed.

What puzzles me is that in safe mode (which disabled addons) does work, but when i disable addons manually, it doesn't! This means to me that the cause is not an addon. It's something else, but i can't understand what. A setting maybe? What else does "safe mode" do, except disabling addons?

The next thing to check is the hardware acceleration setting:

https://support.mozilla.org/en-US/questions/1309612#answer-1360188

The mozillazine article cited above explains the effects of safe mode.

Well, i found it, disabled it and restart. No change in TB's behavior. Anything else i can check?

Are you using a non-default theme in normal mode, or any custom css files?

Nope. I've used custom css in Firefox only, not Thunderbird. Also, the theme is the default.

Are there perhaps any related "advanced" settings i could check?

At this point, it's probably faster and more effective to create a new profile, add an account, and see if it works properly. Help/Troubleshooting, about:profiles, to create and manage profiles. Your current profile is unaffected and can be run from about:profiles. If the new profile works, data from the old one can be transferred to the new.

Consider also updating to 78.4 (check Tools/Options/Advanced/Update), as 68 is essentially end-of-life.

I will try that, thanks.

You said: Consider also updating to 78.4 (check Tools/Options/Advanced/Update), as 68 is essentially end-of-life.

!!!!!!! Why didn't it automatically update to the newer version? I have "auto-updates" turned on and pressing "check for updates" reports "TB is up to date"!

Auto updates have just resumed with the release of 78.4. Because of previous problems with the 68 to 78 process, auto update was suspended, but is now being deployed gradually. You can install 78 manually, or wait until auto update reaches your setup. If you install 78 manually, I recommend starting a new profile with 78 anyway.

I see. Thanks for the info and your advice. I'll update to 78 and hopefully the problem will be fixed.

Is it possible this is an interaction with a spam filter or something from an anti virus. Thunderbird safe mode disabled the addon interfaces to create the mode as I understand it. So external software that is using them, but not registering their addon as installed would leave mystery changes that are difficult to explain.

Well, i tried disabling all relevant options (spam, antivirus, junk) and antivirus itself, but nothing changed. As i was trying various stuff though, i noticed that TB was automatically upgraded from 68.11 to 68.12.1 and all of the sudden, the problem disappeared!

I re-enabled everything and it now works as expected! I'm guessing it was a bug /side-effect of something, which was somehow fixed. As i see now, TB is also offering to be upgraded to 78.4, so i'll go ahead and do that too.

Thanks to everyone for your help and ideas.

Please note that some chnages occur when you update to 78, so before updating I would advise you exit Thunderbird and make a backup copy of your current profile name folder - just in case you want to go back and put it in a folder that has the name of the last version used on that profile eg: TBv68.12.1 Then go ahead and update.