[GTALUG] (question) how to preserve font in Thunderbird

To those using Thunderbird... Thunderbird is set to use "Variable Width" font and "medium" size.     - When I write, it looks like "Noto Serif".     - When I see my post, either in "Sent" folder or from GTALUG list, the font is not what I sent out.  It looks like some "san serif".  And, also, size is not right. So, how do you preserve font/fontsize in Thunderbird? --William