You are wasting your time posting this. They do not care. There is nothing professional about Thunderbird and the way it mis-handles font size. I have complained to them about this for years and the only response I get is a complete joke. They act like they have no idea that font size is normally designated with numbers that actually mean something. To make matters worse, the settings page actually asks what font and what number size you want as your default. But, obviously, those settings were never implemented and all we get when composing an email is the completely meaningless small, medium, large. blah, blah, blah. And the font size will change in the middle of your message, without your knowledge, and the only way you find out is when someone replies and you finally see what they saw when they received your email. It makes you, the Thunderbird user, look illiterate and completely unprofessional.
It is quite clear that whomever is developing Thunderbird is incompetent and actually is illiterate. Either tell us you don't care about professionalism in your product or fix this forever problem. Meanwhile, Thunderbird deserves to be ridiculed and denounced online so that no other professional person ever makes the mistake of thinking their emails will look professional if they use this poorly designed program.