It is in fact a known bug, caused by versions of fontconfig < 2.13. See https://bugzilla.mozilla.org/show_bug.cgi?id=1566053 and https://askubuntu.com/questions/1076412/firefox-freezing-with-100-cpu-usage-for-30-seconds-when-launching-chromium#answer-1089841.
It is in fact a known bug, caused by versions of fontconfig < 2.13. See https:/ /bugzilla. mozilla. org/show_ bug.cgi? id=1566053 and https:/ /askubuntu. com/questions/ 1076412/ firefox- freezing- with-100- cpu-usage- for-30- seconds- when-launching- chromium# answer- 1089841.