This link has the information, although you may need to experiment a bit for what you prefer: https://support.mozilla.org/en-US/questions/1462509

Thanks for your prompt reply, but I still cannot get the font sizes to change. When creating a text, the font is so small I can hardly read it. The only way I can increase the font size is using the "bigger" tools in the text control at the top of the message. When I do that the size is acceptable. But when looking at a many years sent message to my horror the text the recipient sees has also increases its size. Only discovered this when I opened a previous sent email. I'm sure my email messages are he laughing stock of the company.

I have tried specifying a 14 pt and 17 pt size your settings but they do not seem have any affect. I have been using computers for many, many years (1976) and used MS Outlook until they tried to force me to upgrade. Don't ever intend to return. I was really happy with Tbird until this Bucropped up. Hopefully it is something I have done that caused the problem But it is consuming too much of my time to ignore, Got to do Something

Post the same screenshots that are in the link I provided. I think that may allow some suggestions. thank you

Here are the screenshots for anything listing fonts in the settings menu

i suggest putting a number on minimum font size for other writing systems and setting font size to 20 for all to see if that has an effect.