Xfinity web page takes over browser since Yosemite upgrade

Right after I upgraded to Yosemite, a web page ad for Xfinity WiFi popped up on its own. I closed it and thought nothing more of it. Later when I was browsing on Safari the same Xfinity ad popped up again. This time I tried to go to another web site but the Xfinity page keeps coming back. No matter how I try to change to another web site, the Xfinity page comes up. The only way to get rid of it was to use Time Machine.

Now I just installed a recent upgrade for Yosemite and am now having the same problem with the Xfinity site taking over my growers.

Has anyone else experienced this?