IE has a pop-up blocker installed. Unlike IE, Fx's pop-up blocker is built in. Firefox can tell when you physically click on a link, thus allowing that pop-up to occur. IE's pop-up blocker, however, cannot tell when you physically click all links. Cause of this, it just blocks all pop-ups unless you click a link while holding a "by-pass" key (generally Control).
yerba# rm -rf /etc