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