I had this happen a few minutes ago. I think it was for an ad by a paint company (Sherwin Williams?), but by the time I located which window it was and scrolled back up to the top, the ad was just finishing squawking. I (or my mouse cursor) definitely wasn't anywhere in the vicinity of the ad when it started blaring. Often I have several TD tabs open in Firefox, making it hard to locate which one is the culprit.