UPDATED to include the image :) If you are talking about only reading emails after you open to read or write. This can be set up: * 'Menu icon' > 'Options' > 'Options' > 'Display' > 'Formatting' tab * Choose prefered font. * Choose size * Click on 'Advanced' button * Set all the sizes to prefered size. * Click on all the 'OK's see image below. There is an Addon which offers more choices on font control in general - BUT there have been recent reports that this is addon needs updating. However, there has been a recent update only a couple of days ago. Version 64.1 * https://addons.mozilla.org/en-US/thunderbird/addon/theme-font-size-changer-for-tb/ download the addon to your computer. then install the addon following this information: * https://support.mozilla.org/en-US/kb/add-ons-and-extensions-faq#w_how-do-i-install-an-add-on