Configure Thunderbird for HiDPI/Retina displays

Mozilla's Thunderbird email client is a great piece of software with a great community behind it. One of the things that did not work well for me out of the box, though, is the small default font size in Thunderbird, which creates problems on high-res “Retina” displays. Here is how you adjust the defaults.px.gif

The way to go is to tell Thunderbird to use a larger font size for the the message composition window and the message display window when working with plaintext emails. To do so, first open Thunderbird's “Preferences” via the “Edit” menu (or the “Thunderbird” or “Tools” menu, depending on your system). Then open the tab “Display” and the sub-tab “Formatting”. There, you simply need to select a larger font size than what is set by default. For me, 16pt works well on a 1920x1080 screen.

Now navigate to your Thunderbird profile folder, which will usually be something like ~/.thunderbird/<Profile name>/ on Linux and macOS, or something like C:\Users\<Windows user name>\AppData\Roaming\Thunderbird\Profiles\<Profile name>\ on Windows. The profile name is usually a string of letters and numbers, followed by the word .default. Open that folder and create a file userContent.css, if it does not exist already. Open the file and paste this into it (again, feel free to change the font size if another number works better for you):

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

This should do the trick.