Ubuntu – Fonts in message area too small to read in Thunderbird

You said there was a problem during the upgrade? To make sure the upgrade is totally finished I suggest you try:

sudo dpkg --configure -a

Or try to reinstall Ubuntu fonts:

sudo apt-get install --reinstall ttf-ubuntu-font-family

You can also try to rebuild the font information files:

sudo fc-cache -f -v

Also if you have a .fonts.conf file in your home directory backup it and delete it or just move it:

mv ~/.fonts.conf ~/fonts.conf.inactive

After some searching on the net, I found the solution to my own problem. I include a description of it below to hopefully save others with the same problem some effort:

At the top of the dialog window under Edit > Preferences > Display > Formatting > Fonts > Advanced, there is a drop-down menu labelled Fonts for. This is initially set to Western, meaning all settings below only applies to messages with those encodings. To change fonts for messages with utf-8 and other non-western encodings, choose Other Languages in the drop-down menu then set the font types and sizes to the same as you have for Western.