For unknown reasons, FastMail's Settings page offers more "Text Size" options if you're using Google Chrome than if you're using Mozilla Firefox. The "Text Size" option is saved on the browser side, not on the server-side. This means that if you don't keep cookies or sometimes use a different device (such as a public computer), you'll have to reselect "Text Size" in future sessions (unless you want to use the default). I use a font from
upfonts.com since I really liked watching the storied toy as a kid.