Mozilla Thunderbird by default use Times New Roman font when you compose message. Those who want to change default font and size while composing messages this article may help you. This article written for Mozilla Thunderbird 3.1.6. Some step may be different in other versions. First of all you need to enable HTML messages from tool preferences. Here are the steps.
Go to –> Tools –> Account Settings
In the dialog box select desired account and then select Composition & Addressing under selected account name. In the Composition & Addressing panel select “Compose messages in HTML format” check box. Click on OK button to close dialog box.
Now Go to –> Tools Â –> Options
In the next dialog box select Composition tab and then select General tab. In the General tab change font name to desire font name. Also change font size, color and background color for the font. Now click on Send Options button.
Select Send Email in both plain text and HTML from drop down and click ok button. This option will enable other side to see email in plain text if there email client not capable in showing emails in html format.