Note: I cleared my History but that didn't seem to work, so I decided to check out preferences.
This worked for me:
1. Go to the Safari menu (top left) and select "Preferences..".
2. Click on the "Advanced" tab.
3. Deselect (uncheck) "Stop plug-ins to save power"
4. Make sure that Xfinity.com is set to on in the Websites tab, for the Adobe Flash Player. (update Adobe Flash Player if it hasnt been updated in a while -- I did not update mine v32....
5. Restart Safari; and go check the streaming for your channels.
... View more