A lot of people have been reporting a new breed of pop-ups on the web. This increasing menace is rooted in the pop-up capabilities of plug-ins like flash and Java. If you're seeing pop-ups and pop-unders, you're probably visiting sites that have flash or other plug-ins and those plug-ins are being exploited by advertisers to abuse you with annoying pop-ups and pop-unders.
Firefox has the capability to disable these pop-ups but it wasn't enabled by default in Firefox 1.0 because we had concerns about websites that rely on plug-in triggered pop-ups for legitimate functionality. Give that these pop-ups and pop-unders have really started to spread on the web, we're testing a quick patch that enables the Firefox pref to block them.
News source: Asa Dotzler's (Mozilla Developer) Blog