Configure Thunderbird for HiDPI/Retina displays

Mozilla's Thunderbird email client is a great piece of software with a great community behind it. I recently switched back to Thunderbird after the developer community fixed a long-standing bug that made working with East Asian languages (Chinese, Japanese, Korean) pretty much impossible (see their papercuts project). Also, since Thunderbird now supports maildir by default, it has become a lot faster and feels much more lightweight than many of the alternatives. One of the things that did not work well for me out of the box, though, is the small default font size in Thunderbird. This was a minor annoyance on regular “low-res” displays, but now that we have high-res “Retina” displays, I needed to find a way to adjust the defaults permanently to be able to read emails.px.gif

The way to go is to tell Thunderbird to use a larger font size for the the message composition and the display windows when working with plaintext emails. To do so, first open Thunderbird's “Preferences” via the “Edit” menu (on macOS this may be found in the “Thunderbird” menu, on Windows this might be in the “Tools” menu). 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. As mentioned before, Thunderbird itself should respect your display settings and any scaling factors you apply for a specific display (e.g. on Ubuntu and derivatives, see System Settings –> Display and set the “Scale for menu and title bars”), so no further adjustments should be necessary here. If you still want to change the scale of the Thunderbird window as a whole, however, find the “Config editor” from within the “Advanced” tab in the Preferences windows, then search for the value layout.css.devPixelsPerPx and set it to something larger than the default. The ArchWiki says a value of 2 works well on Retina screens.