The font you choose in: 'Tools' > 'Options' > 'Display' > 'Formatting' tab ....will be used as default font for emails you see and emails you Write as they appear to you. See first image below as example. click on 'Advanced' to set minimum font sizes. you will also see that here is also a selection to 'allow messages to use other fonts', if the font has been hardcoded into the html. This setting in Display, the font is not hard coded in 'Write' new emails, it only looks like that from your view point. This is a good idea, because hardcoding a font may make your email unreadible or look messy on another OS that does not have those fonts installed. ------------------------ Tools > Options > 'Composition' > 'General' tab see second image below. Font: should ideally be 'Variable width' and size 'medium'. It is possible to set a default font for composition but it does have some drawbacks. The settings in this section can affect how messages are sent. If you make unusual choices, then people who receive messages from you might find them difficult or impossible to read. see more info here: http://kb.mozillazine.org/Font_settings_in_Thunderbird You can also select a different font at time of writing using the 'Formatting Bar' options. This does set a html font as hard coded in the html, so the settings are sent with the email. It also is only set for that email. It is also worth noting that if anyone reads your emails in Plain Text then all formatting is removed. It is also worth noting that no matter what you choose, the recipient can control how they view the email.