How to make Thunderbird compose mail with “Paragraph” as default?

I have found that to set the default to "Paragraph" or "Body Text", you need to navigate from the main menu:

Tools->Options->"Composition" region -> "General" tab

In the HTML section, find the option for "When using paragraph format, the enter key creates a new paragraph"

Select that option if you want "Paragraph" as the default. Deselect that option if you want "Body Text" as the default.

I am using Beta version 45.0 of Thunderbird

In Thunderbird 45+ you could change between "Paraghraph" and "Body Text" style from

Tools -> Options (Preferences) -> Composition -> Check/Uncheck the following option:

"When using paragraph format, the enter key creates a new paragraph"