I don't think I'm the only one who finds the font settings in Thunderbird confusing especially to a new user. I learned through Reddit to set fonts you go to Settings/General and have a default font to set, than in Advanced you choose "Latin" and set font styles there, then you choose "Other Writing Systems" and choose font choices there as well. I don't actually know what font settings these control, or as a new user how I would know to adjust these. I also don't know how these interact with choosing a font when composing a message. I could not find any information source that explains why fonts are set up this way or a guide on how to set up fonts properly. I never had this issue using other email program such as Outlook, eMclient or BlueMail.
My idea to put forward is to revamp the font setting approach in Thunderbird to make font settings more intuitive, use plain language and/or to provide a guide to managing fonts in the program.