There have been numerous complaints about an Xfinity "popup" that continuously appears in various locations on the screen. Here is a solution I have found for computers running the Mac OSX:
1. go to System Preferences by clicking the Appple in the upper left-hand conrner of the screen
2. select Network
3. Look for and select Wi_Fi in the list
4. In my case, the Network name was Xfinity.
5. use the pulldown menue and pick a different WiFi. In my case, I slected the WiFi name for my own router at home.
6. If you do not see a network that is yours, or that you want to use, you can delete (turn off) WiFi by hitting the minus button at the bottom of the list.
Hope this helps.