Flatpak Firefox/LibreWolf Square box for text - How to fix

Error

Text in a browser, such as Firefox or Librewolf is not displaying correctly, for example with no anti-aliasing.

Solution

Firefox:

cp /etc/fonts/conf.d/*.conf ~/.var/app/org.mozilla.firefox/config/fontconfig/conf.d/

LibreWolf:

cp /etc/fonts/conf.d/*.conf ~/.var/app/io.gitlab.librewolf-community/config/fontconfig/conf.d/

Extra Info

If these folders do not exist in the Flatpak app folder, create these.

Sources

https://github.com/flatpak/flatpak/issues/4571#issuecomment-977935945