I was used of fonts in Thunderbird that are thinner and more readable on Windows than in Ubuntu.
To get better fonts for Thunderbird 68.4.1 I added a chrome folder in my thunderbird profile folder
You can find this folder zipped here: https://drive.google.com/file/d/1LdVD4ZI5A19ALWr644cx_53vHwvK5MCA/view?usp=sharing
If it's written in black in the left panel, change the colour from Thunderbird - Themes.
Default font is too condensed, the priority is not the beauty but the readability...