Beginning with Firefox version 52, the login page will display this warning unless you are using HTTPS to access Netreo.
You can use HTTPS to avoid this problem. However, by default Netreo ships with a self-signed certificate which will then cause a “Your connection is not secure” error when you attempt to access it.
You can install a valid certificate for your server if you choose to purchase one to prevent this from happening. See, How to Install Your Own SSL Certificate.