Stefan Angrick

Mozilla's Thunderbird email client is a stellar piece of software backed by a great community. But one of the few things that don't work well for me straight out of the box is its default font size. The default font is rather small, which can be a problem on high-res “Retina” displays. Here's how you adjust the default settings.

The trick is to tell Thunderbird to use a larger font size for both the message composition window and the message display window when dealing with plaintext emails. Start by opening Thunderbird's “Preferences” via the “Edit” menu (or the “Thunderbird” or “Tools” menu, depending on your operating system). Then head over to the “Display” tab and pop into the “Formatting” sub-tab. Here, simply select a font size that's more to your liking. From personal experience, 16pt works works well on a 1920x1080 screen.

Now navigate to your Thunderbird profile folder, which typically resides at ~/.thunderbird/<Profile name>/ on Linux and macOS, or at C:\Users\<Windows user name>\AppData\Roaming\Thunderbird\Profiles\<Profile name>\ on Windows. The profile name usually consists of a string of letters and numbers, followed by .default. Open that folder and, if it's not already there, create a file named userContent.css. Open the file and drop this line of code inside (feel free to tweak the font size if you fancy something different):

* { font-size: 16px !important; }

That should do the trick!