It's encrypted for me. Must be something in your local settings or certificate handling. I'm running Win 10 64b and entering my.xfinity.com on Edge, Chrome and IE, all jump to https://my.xfinity.comand and are secure. Firefox, however shows unsecure.
What you are seeing is an indicator from the browser that one or more items on the webpage from other sources isn't using https, but http. The "My Xfinity" homepage itself is secure. It could be a link, an image or any number of other things triggering the warning. Here is the Firefox explanation of how to judge "mixed content" on a webpage--------------------