Examine your settings in OPTIONS - DISPLAY - ADVANCED.

The correct location for mark as read options is:

Press the alt key to make the Menu Bar appear. if not showing. From the Menu Bar select Tools-Options-Advanced-Reading and Display

@ Airmail. I was referring to the Desktop version. Not familiar with mobile menu options so thanks for adding this info.

I am using the windows desktop version The mobile version of Thunderbird is software by other authors and is not supported here.

If I follow your menu path shown above it takes me to a screen to set Fonts & Encodings.

Hmmm.... PM'd you as I do not want to go off topic nor do I wish to bother poor Mister Digital anymore. :)