Killed by a popunder ad

I'm compiling a list of resources for customers of a major ISP. The kind of thing that will be displayed when they use the bundled CD to install their internet connection.

Anyway for the reference section, I needed something more than just Wikipedia (though I don't need anything more myself!) so I thought I'd put in the Encyclopedia Britanica. Then I noticed Firefox telling me it had blocked a popup from that site. Helpfully called "popunderAd". Well guess what Britannica, I won't be sending anyone your way with that kinda behaviour!