Computer Configuration > Administrative Templates > Firefox
Both
HKLM\Software\Policies\Mozilla\Firefox
If this policy is enabled, you can define a list of external protocols that can be used from listed origins without prompting the user. If this policy is disabled or not configured, any site that invokes an external protocol will ask the user for permission. For detailed information on creating the policy, see https://github.com/mozilla/policy-templates/blob/master/README.md#autolaunchprotocolsfromorigins.