I assume you're writing in plain text. To ensure your text is the right font and size, there are a few steps Okay, this may sound strange, but here is the real way to set fonts on Thunderbird. And I attached examples of my own setting of Calibri 14 as my default. Here goes:

- in Tools>settings>general, set your default font. 
- then click the advanced button. Notice the drop-down menu at top. It probably says 'latin' or 'other writing systems' - this is IMPORTANT: 
- select 'latin from that menu, set your desired font and font size settings, click OK, 
- select 'otherwritingsystems' from the drop-down menu and set the IDENTICAL settings to what you did for Latin, and then click OK. In attached screenshots you will see my latin and otherwritingsystem match.  Notice I also set monospace font.
- and, if you have account set to compose HTML, then set that in tools>settings>composition to variable-width and medium font. (You would only change the HTML font setting if your intent is for recipient to receive your preferred font - not my personal recommendation.)
- Set Sending Format to 'automatic' 

With these settings, your compose window and your sent folders will match. And if you don't write HTML (e.g., bold, italic, color), none will be sent . If you do these settings once, your font will always be consistent. This does not protect you from someone sending you a message in some god-awful font with 30 pt size, but this does provide consistency.

Note: The settings are pixels, not points, so you may need to set a higher number than anticipated.

Thanks David I'll do as you suggest. sue

Variable fonts are a relatively new technology that allows for the adjustment of various attributes of a font, such as weight, width, and slant, in a seamless and continuous manner. These fonts offer greater flexibility and control over typographic design compared to traditional static fonts.

When it comes to using variable fonts in email, there are a few considerations to keep in mind:

Email Client Support: Variable fonts are not universally supported across all email clients. While some modern clients may support variable fonts, others may not render them correctly or at all. It's crucial to check the email client compatibility before incorporating variable fonts into your email design.

That comment might confuse the original poster. In Thunderbird, the selection of 'variable fonts' in composing HTML message means that NO FONT selection is embedded in the outgoing message. This allows the recipient's email client to define the font that is used. Thank you.