Unwanted Google ads on wordpress.com blogs

  I just noticed that there's a Google ad on a recent story on my mother's wordpress.com blog. Here's a screen shot. Every story that I clicked on on her site now has an ad. This is new.
  I called to find out if she had done anything to have these ads appear, and she said no.
  Then I went to my Droidie site, and it has ads now. Oy.
  But not on Unberkeley.
  Thankfully not on the memorial site for my father.
  No ads on Frontier News.
  Maybe I missed something, maybe there's an opt-in that we checked off by accident on the sites that have ads.
  If there's some kind of revenue share, it would be nice to get the money, but as I've said before, many times, I don't want ads on my sites.
  What's going on??
  Update: There is a disclaimer on wordpress.com. "To support the service (and keep free features free), we also sometimes run advertisements."
  Update #2: There's more. See item #2 on this page. If you're using Firefox (I am) you won't see the ads. So there's a bug. But actually I'd rather they not hide the ads from some users. That's probably why I wasn't aware they were showing ads to some of my readers, because I've been using Firefox for years.