I came across this thread after experiencing a similar problem while using Firefox. Since this thread hasn't been solved, I thought I'd chime in. Over roughly the past week, two of these pop ups appeared. Not giving much thought on the first occurrence, I didn't recall the site I was visiting at the time. Today, it appeared while visiting the website of a local newspaper, which has a lot of banner ads. I selected "cancel" both times. Not sure if this is related, but I think I noticed a criteo link among the several that flash in the status bar while the page is loading.
I've provided a screenshot of what it looks like below. After doing some of my own investigating in online forums, I have yet to come to a conclusion as to what I have been experiencing should be of concern (e.g. malware) and require further action. After the first instance, I cleared my cookies as some have recommended, although the pop up has since resurfaced. I even explored the AdBlock add on, but didn't see a place to enter a web address and don't know if this method is effective. The strange thing about this is that I've never seen this particular pop up during the years I've visited the site or while I've owned my Macbook Pro. Any input would be greatly appreciated and I apologize in advance for my elementary understanding of the subject. Thanks!
