So, when I go to Merriam-Webster's dictionary in Firefox, I get a pop-under ad. When I go to the same site in IE, no pop-under. Here's the link I tried. Same link in both browsers.

I think this is interesting from an economic perspective. I'll list a few options I just thought up myself:

• Could it be that Firefox has sufficient market share to make it the primary target of nefarious advertisers? They're more motivated to locate and exploit non-IE-browser-specific holes? That is, have they perhaps lost income because more folks are using Firefox and not seeing their popups, so they were motivated to find and exploit Firefox-specific holes?
• Could it be that IE's status as primary target by nefarious advertisers has paid off? That the market has policed itself and Microsoft's response has been adequate — and since Firefox hasn't had this level of real-world use and testing, a shortcoming was found and exploited?
• Pure chance occurrence; the pop-up dudes got lucky on my config.

Note that I only have out-of-the-box popup handling on Firefox; no extensions dealing with popup blocking. Google toolbar is installed on IE but the popup blocker is turned off; perhaps that may have something to do with it. I dunno. But it seemed strange; I can remember when "popup-killing" was one of the killer features of Firefox.

I'd guess it is a combo of the first two options above.