Pop-ups

Those persistent pop-ups that get past your pop-up blocker are, by and large, triggered by the Flash plug-in. According to Asa Dotzler, there is a hidden switch in Firefox that you can use to block them, though they didn't leave it on because of the possibility that people are using legitimate Flash pop-ups.

(Personally, I don't think I've ever seen any legitimate Flash pop-ups unless you count gags on Homestar Runner.)