Thunderbird is rich in its offerings for fonts, and it is important to follow the steps to get exactly what you want. I put the steps in this prior post:

  https://support.mozilla.org/en-US/questions/1462509     

Note: Remember that your font settings can be for you AND recipients, or just for you. The indication in my article is that fonts are for you, the sender, as the recipient may not appreciate your preferences. It's your decision, obviously. Good luck. Let me know how it goes.

Hi David, Thanks for your reply. 1, 2 and 3 done. Can't find #4. Other writing systems. Please assist. Many thanks:)

Mary, it's the top row on that pane where it says 'latin' - click that and then select the 'other writing systems' option, possibly at the bottom of llist - and set ALL options to match what you set in the latin pane. Let me know how it goes.

Success! Sorry I'm a bit slow... Many thanks David:)

Actually, you're quite fast. Sometimes, it takes ten or more interactions to resolve this. Enjoy. You're very welcome also. We're glad to assist.