First, please revisit your objective. Many people prefer to read email in their own preferred font and in their own preferred size. Forcing recipients to read messages in an undesired font and undesired font size can seriously diminish the content value of the message. Having said my piece, if you are wanting your message to be sent in your font and font size, then you need to set tools>settings>composition to a font other than 'variable width' and if you want it in a different font size, you will need to change the default there. By default, Thunderbird sends HTML messages if the message includes styling, such as bold, italic, color.

Thank you David, good point, well made. But my dilemma may arise out of a much more fundimental problem - that I can't find a relevent answer to elsewhere In 'Support'.

Which is, that until quite recently, all my mail folders contained mostly messages in HTML format - if by HTML I mean that all the inserted formatting, logos, links and colours were retained as sent by Commercial responders as well as from my Private contacts.

But now, my complete Message Store (including Inbox and Sent) has reverted to monochrome, unformatted Text - except for the (blue, underlined) fully active links where relevent.

Do you have any helpful guidance on this perhaps?

OK, the issue is different than originally stated (or understood). My first thought is to look at View>Message Body as>original HTML.