After getting an new 4k monitor you may wounder how to adjust the font setting of Thunderbird. May the font is a little bit to small. So you will go and search for the right setting. But there isn’t one in the UI. You need to use the configuration editor. (about:config). So open Options, scoll down to the end and open the „Config Editor“.
Now you can adjust this setting:

layout.css.devPixelsPerPx

Per default it is set to -1. Which means „use the system settings“.
But if you do not want to adjust the system settings for some reasons you can override it.
A value of 1.5 will scale the display by 150%. A value of 2.0 by 200%.
Find the setting you like 🙂