Comcast is taking to injecting pop-up ads into the browsers of customers who have their own router in order to "encourage" those individuals to rent a router from them so they can enjoy "the full benefits" of their Xfinity service. A sales tactic that goes a bit too far and is most definitely a huge privacy concern.
In the past they've injected pop-ups to warn users of copyright infringement, which isn't necessarily a terrible interruption if one isn't quite aware of that infringement, though it can certainly still be concerning when the injection of anything can be seen as malicious in and of itself.
Now, however, Comcast is apparently plugging in pop-up ads into your online experience to get you to "upgrade" your router to one that they provide, which they do make a pretty penny from. It used to be that you'd receive an influx of phone-calls asking you to "review" your account, or perhaps some actual physical mail, but now it's much worse.