It also appeared on Safari on my iPad!
I've seen this on my iPhone. The spinning 'Amazon wheel you won' BS. Does it happen with the same frequency on the iPad?
What happens is these poisoned ads get injected into a perfectly legit web site's ad rotation, the web site realizes it, then they get rid of it but it may take a while. For example I use http://wordplays.com
for my crossword hints because it is lean and mean. They have few ads but not much. I never use it on the Mac, just my iPhone. A while back sometimes when I'd go there it gave the Amazon BS. It went away a few months ago.
Maybe it's some site you go to on both the iPad and Mac?