Fix for funky fonts in Firefox Flatpaks.
Add the following to ~/.var/app/org.mozilla.firefox/config/fontconfig/fonts.conf and reboot. The file (or fontconfig) may not exist so go ahead and create them.
<?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>