When I modified my webpage to add the Firefox Referral program, the same thing happend to me. But when I put the mouse over that ad, it says go to Yahoo.com in the status bar! Do you think that when can not display the referral ad, it shows some other thing like this?

could be, but mine was showing both the firefox referral ad and that weird ad. is yours still showing it?

First, I added the script and some regular ads appeard instead of the firefox bar, so I commented it because I thought it was not working right. And just after adding the comment tags around the script (with <!-- -->) that new type of ad was on my page.