Computer Configuration > Administrative Templates > Firefox
Both
HKLM\Software\Policies\Mozilla\Firefox
Note: In order to use this policy, you must clear all settings in the old Preferences (Deprecated) section. If this policy is enabled, you can use JSON to configure preferences. If this policy is disabled or not configured, preferences are not modified. For detailed information on creating the policy, see https://github.com/mozilla/policy-templates/blob/master/README.md#preferences.