It's an annoyance of a different sort.

I was reading a thread, and scrolled to read earlier posts. Just before the Delta advertisement went off-screen, Firefox became unresponsive for several (10-15) seconds. This was consistent whether I scrolled (using a scroll ball) or jumped (using the scroll bar), and it occurred again when I closed the tab.

If you can figure out what the Delta ad is doing, please make it stop.