Fix Firefox Font Issue in Flatpak
Disable the bitmap fonts by putting the following in ~/.var/app/org.mozilla.firefox/config/fontconfig/fonts.conf
file:
<?xml version='1.0'?>
<!DOCTYPE fontconfig SYSTEM 'fonts.dtd'>
<fontconfig>
<!-- Disable bitmap fonts. -->
<selectfont><rejectfont><pattern>
<patelt name="scalable"><bool>false</bool></patelt>
</pattern></rejectfont></selectfont>
</fontconfig>
Source: Flatpak GitHub