Under Settings > Composition > HTML Style I have Font set to Courier as I find it the easiest to spot typing errors in and it also has a comprehensive set of glyphs. Despite this Thunderbird does it’s best to switch it to ‘Variable Width.’
If I copy text from a previously sent message that was itself in Courier and paste it into the Composition window it will paste as Variable Width. If I use Insert > HTML and then resume normal typing that too will be in Variable Width.
I believe that having set a font in settings that is the one that should be used unless explicitly instructed otherwise.