So you have some fonts that don't appear in Thunderbird, and you ask how to install them. What do you imagine will happen when a correspondent receives a message from you which calls on one of these fonts? You had to install them. Why would you expect any other users to have them? For the best chance of your messages appearing to your correspondents as they appear to you, stick with the so-called "web safe" fonts. I haven't tried them myself yet, but Google Fonts look promising as they will be dynamically downloaded on demand. But you'll have to create css style definitions and the only way I would be confident about to use those in Thunderbird would be to set them up in a Stationery template. It's possible you could also do it in a template or in a userContent.css file..