Yosemite Security Defaults
Refer to this article for more information about the Firewall and how to enable it: https://support.apple.com/kb/PH18646?locale=en_US

It is fine if the main computer user is an admin. You will still need to enter your password to authorize changes to the computer, the only thing that is different from being a standard user (in this scenario) is that your username will be automatically entered. As an administrator, you also cannot delete important system files (built-in apps, operating system files, etc.).