How To Make Thunderbird Compose Mail With "Body Text" As Default?

There is an option that controls the behavior in the preferences under "Composition" → "General"

Use Paragraph format instead of Body Text by default

To compose in "Body Text" mode by default, you need to make sure that option is unchecked.

This dialog is accessed differently on different platforms:

Edited: On older versions of Thunderbird the checkbox was labeled "When using paragraph format, the enter key creates a new paragraph"

While creating a new message, click in menu Format » Paragraph » {whatever you want}.

This will become default style for new messages.

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