Pop-up from http://security-scan.xyz

I am on a MacBook Pro using Yosemite 10.10.2 and an updated version of Safari. I am currently using Firefox. When I open Safari, a pop up from http://security-scan.xyz comes up with a Firewall Warning saying I need to call a toll free (855) number to take care of network damages. I assume this is a non Apple company. I have tried quitting Safari and restarting. Any suggestions? Has anyone had this issue before?