Okay, this may sound strange, but here is the real way to set fonts on Thunderbird. And I attached examples of my own setting of Calibri 14 as my default. Here goes: - in preferences>general, set your default font. - then click the advanced button. Notice the drop-down menu at top. It probably says 'latin' or 'other writing systems' - this is IMPORTANT: set your desired font settings in one, and then set the IDENTICAL settings in the other. In attached screenshots you will see my latin and otherwritingsystem match. - and, if you have account set to compose HTML, then set that in settings>composition to variable-width and medium font. - while there, click 'send options, and check send plain text when possible (if that is your intent). With these settings, your compose window and your sent folders will match. And if you don't write HTML, none will be sent . If you do these settings once, your font will always be consistent. Set color in tools>settings and click the advanced tab. be sure that 'use system colors' is NOT ticked.