I have seen this rendered on the Gecko engine (Firefox, Orca). I use Avant/IE (Trident) with a fairly large (but very good) HOSTS file (with a very simple CSS to remove the top banner) and have not had a problem.
You can right-click on the Properties of the ad and find out where it is served. I am fairly sure this is something TNT would be aware of right away and be working on fixing (they are usually at least very good at making sure their ads FIT the places they are meant to be on the site; a skyscraper ad in that area is wrong). It probably requires the cooperation of the company that hosts the ads. Be patient. It is still the weekend NST.
In the meantime, search PPT for "recent" threads about HOSTS and the viruses in one ad server (that may be gone now or not; I have heard no response, but have also taken the precautions to protect myself anyway).
One odd ad I have noticed recurrently is some Attack of the Slorgs ad, that apparently isn't Flash and so therefore I don't know why it is not blocked when I have Flash disabled. (I don't think it's an ani-gif either, that is why this game ad in particular is so mysterious to me as it is not hosted ON neopets, and if I block the adserver, it still appears).
Anyway, good luck ridding yourself of the ad. Seems ironic lately Firefox users are more prone to the quirks of Neopets.com recent revamps. Remember Neopets was coded in and for IE. Sorry, but we have the market share, despite all the "problems" some people have using it.
EDIT: Actually the Slorg ad has been gone now for a while. Eventually my software "learned" it, just to clear that up, no issues otherwise now.