Computer Configuration > Administrative Templates > Firefox
Both
HKLM\Software\Policies\Mozilla\Firefox
If this policy is enabled, you can define a list of domains and file type extensions that will be exempt from executable warnings. If this policy is disabled or not configured, warnings are shown for all executable file types. For detailed information on creating the policy, see https://github.com/mozilla/policy-templates/blob/master/README.md#exemptdomainfiletypepairsfromfiletypedownloadwarnings.