The composing of emails offers a lot of options via the Formatting Bar or via Menu Bar 'Format and 'Insert' options but it's not to be compared to a Word Processing Program because it's available code just not that complex. A Word Processing Program can control more precisely such nuances as line height, but and this is a big difference - people read those documents using the appropriate program eg: OpenOffice Word, OpenLibre Word or Microsoft Office Word.

Those kind of documents would be sent as an attachment in an email so the receipient can choose appropriate program to open them.

When you are composing an email, remember in reality what you see on your computer when composing an email, may not be what your recipient sees. Different OS may have different fonts installed. Monitor screens have different resolutions. Not everyone will view in HTML. People can choose to alter the font and pixel size.

Thunderbird already auto adjust the basics when it comes to line height. Choosing a different font will auto adjust the line height for fonts. Choosing a different size for font - larger smaller will also auto adjust the line height. This is also the same if you use Capital Letters.

If you require extra space between one line and another, - a line break - you click on the 'Enter' to leave a blank line. To auto leave extra spacing when creating paragraphs you can swap from 'Body Text' to 'Paragraph'

In 'Settings' > 'Composition' Under 'HTML Style' section You can choose how Thunderbird composes by default. Either enable Paragraph format or uncheck that option to use the basic 'Body Text' format. Paragraph format will leave more space between end of parargraph and start of another.

Please remember that Thunderbird uses px = pixels not pt = points when it comes to font size.

You can use 'Insert' >'HTML' if you understand and can use css html. https://www.w3schools.com/cssref/pr_dim_line-height.php

Please realise that whatever font or font size you use, when the recipient receives the email they can have preferences that overrule whatever you wrote.

What you term as readibity may not apply to others. As an example - Whatever font people choose to use in emails they compose, if they send to me then I've set up Thunderbird to only use the font I want to see. Whatever font size they choose to use, I also control what I see. The minimum size I will see is 12px, as I'm not interested in small fonts.

You can use Tables to help with layout, this generally works well, but obviously users can force some conditions and of course people can also just use PlainText which alters everything :)

What a terrific answer! Complete, detailed, and expertly covering the entire range of my question.

Many thanks! Koodos all around.