Please check to see if this being caused by an addon extension conflict. start Thunderbird in 'Safe Mode' and test.

In Thunderbird 'Help' > 'Restart with addons disabled'

If it works, then it is possible that one of your addon extensions is causing a conflict. Restart as normal 'Menu icon' > 'Add-ons' disable one at a time and restart Thunderbird. When it works ok, you will know which addon caused the problem.

Perhaps the addon needs updating.

report back on results.

There are no addons installed, but the problem disappeared permanently after only one start of Thunderbird in 'Safe Mode' with addons disabled.

What was changed in 'Safe Mode'?

Safe Mode temporarily affects the following:

• All extensions are disabled.
• The default theme is used, without a persona.
• The Just-in-time (JIT) JavaScript compiler is disabled.
• The userChrome.css and userContent.css files are ignored.
• The default toolbar layout is used.
• Hardware acceleration is disabled.

I would start by seeing if this is being caused by the Hardware acceleration.

'Menu icon' > 'Options' > 'Options' > 'Advanced' > 'General' tab

• under 'Advanced configuration'
• uncheck 'Use hardware acceleration when available'
• click on OK

Restart Thunderbird.

I meant that the problem is gone also in normal mode, since I've started Thunderbird once in 'Safe Mode', but I don't know why.