Set default font for composing new messages in Mozilla Thunderbird:

* Make sure each account is set to composing with HTML formatting.

    * Select "Tools" | "Options" from the menu in Mozila Thunderbird.

    * Go to the "Composition" category.

    * Make sure the "General tab" is selected.

    * Choose the desired font, font size and colors under HTML.

    * Click "Send Options"....

    * Make sure either "Ask me what to do", Send the message in HTML anyway or "Send the message" in both plain text and "HTML" is selected under When sending messages in HTML 

    * Click "OK".

    * Close the configuration dialog.