The settings must be put into (path depends on architecture and branch):
/var/lib/flatpak/extension/org.mozilla.firefox.systemconfig/x86_64/stable/defaults/pref
Currently policies.json to be placed into:
/var/lib/flatpak/extension/org.mozilla.firefox.systemconfig/x86_64/stable/policies/policies.json
Differential Revision: https://phabricator.services.mozilla.com/D168803