Posts Tagged ‘fonts’

Customizing fonts in Firefox on Linux

I recently updated my desktop machine from Ubuntu 18.04 to 20.04. (I tend to be skittish and stick to the LTS releases.) Everything went great, except that when I opened some of my go-to websites (such as GitHub), the fonts just looked… off.

Screenshot of GitHub rendered in Nimbus Sans

After some research, I learned a few things:

  1. Firefox relies on an OS-level utility called fontconfig to choose fonts.
  2. On Ubuntu 20.04, fontconfig has a default config file that automatically replaces proprietary fonts like Helvetica with free fonts like Nimbus Sans.
  3. If a web page’s CSS has Helvetica in the font-family property, Firefox will ask the OS, “Hey, do you have Helvetica?” And the OS will say, “Yep!” and serve up Nimbus Sans.

Nimbus Sans is not an attractive font to my eyes. I would have much preferred an alternative like DejaVu Sans or Liberation Sans (one of which, I’m pretty sure, is what Ubuntu 18.04 was using). So I did some digging.

Surprisingly, I couldn’t find a simple answer to “how do I tell Firefox which fonts to use on Linux?”. The fontconfig man pages are too dense to be much help. But after stumbling upon this helpful blog post, I eventually found a solution.

Create a fontconfig file

First, I created a file at ~/.config/fontconfig/fonts.conf (you may have to create the directory). Here’s mine:

<?xml version="1.0"?>
<!DOCTYPE fontconfig SYSTEM "fonts.dtd">
      <family>Liberation Sans</family>

Basically, this file says, “If you’re trying to render Helvetica, prefer Liberation Sans instead.”

Create a test HTML page

Next, I created a small HTML page that just renders various CSS font-familys, like this:

<h2 style="font-family: Helvetica">Helvetica</h2>
<h2 style="font-family: system-ui">system-ui</h2>
<h2 style="font-family: sans-serif">sans-serif</h2>
<h2 style="font-family: Arial">Arial</h2>

If you’re curious, by default Ubuntu 20.04 will give Nimbus Sans for Helvetica, DejaVu Serif for system-ui, DejaVu Sans for sans-serif, and Liberation Sans for Arial (on Firefox 75).

Then my debug cycle became:

  1. Edit the fonts.conf file.
  2. Reload the page in Firefox (you may have to close the tab and open a new one).
  3. Open the Firefox developer tools to the “Fonts” section.

Screenshot of Firefox DevTools showing the "fonts used" section

Using this technique, I could finally get GitHub (as well as some other sites) to better suit my tastes:

Screenshot of GitHub rendered in Liberation Sans

Here is a zoomed-in before and after (Nimbus Sans on top, Liberation Sans on bottom):

Screenshot comparing Liberation Sans to Nimbus Sans in GitHub

Hopefully this blog post is helpful to someone else trying to figure out fontconfig. Good luck!


This research upended my understanding of how font-family works in CSS. I figured that if a website declares font-family: Helvetica, sans-serif, then it will fall back to sans-serif if Helvetica doesn’t exist on the user’s OS. But given that the OS can lie about which fonts it supports (and thus choose “Helvetica”), this can really change how fonts render.

Apparently fc-match Helvetica is supposed to show which font fontconfig chooses for Helvetica. This didn’t work for me. Running this command (with or without sudo) would always return “Nimbus Sans,” regardless of what Firefox was showing. Clearing the cache with fc-cache -rv also had no effect. Go figure.

Also, the Firefox font settings (Preferences → Fonts and Colors) don’t work. Well, maybe they work for choosing the default sans-serif font, but they don’t help if a website’s CSS explicitly lists Helvetica first. (As they often do, which is perhaps a testament to the Mac-centrism of most web developers. I even found this pattern in my own website’s CSS, which probably dates back to when I was a Mac user.)

Also, be aware that Chrome uses its own system for managing fonts, different from Firefox’s. If you test Chrome, you’ll get different results.