This article explains how to configure the Mozilla Thunderbird email client to use your preferred font, size, and color when reading incoming email. The instructions are similar for all versions of Thunderbird on all platforms.
Thunderbird How to Change Default Email Font
To change the appearance of your incoming email:
From the Thunderbird menu bar, select Thunderbird > Preferences (Tools > Options on a Windows computer).
Select the default font and size you prefer. To change colors, select Colors. When you are satisfied with your choices, click OK to return to the Format menu.
