What are your font settings? At the top right of the Thunderbird window, click the menu button <img src="https://user-media-prod-cdn.itsre-sumo.mozilla.net/uploads/gallery/images/2014-01-10-13-08-08-f52b8c.png"> > Options > General > Language & Appearance > Fonts & Colors > Advanced Please post a screenshot of your settings. https://support.mozilla.org/kb/how-do-i-create-screenshot-my-problem