After the merge of <https://github.com/snapcore/snapcraft/pull/3586>, the Firefox snap will hopefully soon see files in the /etc/fonts/conf.d directory. That directory basically consists of files (or symlinks) provided by packages installed on the host system. While it's possible for a user with admin privileges to add their own fontconfig config files to that directory, the natural place for user specific font configuration is $HOME. The user equivalent of /etc/fonts/conf.d is ~/.config/fontconfig/conf.d . fontconfig reads files in that directory as well as the file ~/.config/fontconfig/fonts.conf . But the instance of fontconfig used by the Firefox snap does not. Even if you can edit ~/snap/firefox/current/.config/fontconfig/fonts.conf or put files in ~/snap/firefox/current/.config/fontconfig/conf.d, your changes will be ignored as soon as the Firefox snap gets updated. This is an Ask Ubuntu question which points at the problem: https://askubuntu.com/questions/1368185/fontconfig-that-survives-firefox-snap-updates So we would need a way to make user specific font configuration persistent. While I wrote this bug with the Firefox snap in mind (since it has replaced the .deb by default in Ubuntu 21.10), I suppose this issue is a general snap issue. As an example it was mentioned for the Chromium snap at <https://forum.snapcraft.io/t/snap-package-cannot-read-fonts-conf/16657>.