I figured out myself that all the Windows updates I had the couple of days, has changed the font in choppy letters. I deleted a couple of them and the problem went away.

Still, i find it's very strange that an update, which is designed to HELP the system, is changing the font. So, the question is: which updates of Windows can I rely on and which can't I rely on?

To be clear: it wasn't just in Thunderbird the font got a chopped appearance. It also happened on websites I visited.

See if the extension 'Change Quote & Reply Format' helps. If you can't find it using the extension search function, download it directly from the developer's web page. https://freeshell.de/~kaosmos/changequote-en.html

I don't know if your solution helps, because I have switched to Linux and I don't have any issues now.

Thanks very much for your suggestion, maybe other will find it useful.