The instructions below apply to Thunderbird for Windows, but the process is similar on Macs.

SelectComposition & Addressingunder your email address in the left sidebar.

see to it theCompose messages in HTML formatbox is checked, then selectGlobal Composing Preferences.

Account Settings in Mozilla Thunderbird

SelectGeneralin the left sidebar, then scroll down toLanguage & Appearanceto adjust the default font and text size.

SelectAdvancedunderFonts & Colorsfor more options, including fonts for different languages.

exit the Thunderbird controls windows.

Composition & Addressing in Mozilla Thunderbird Settings

The change takes effect the next time you compose a message.

Compose messages in HTML format and Global Composing Preferences in Mozilla Thunderbird Settings

Language and Apperance in Mozilla Thunderbird settings

Format in the Mozilla Thunderbird message composition window