Popups are almost dead. More and more web sites use absolutely positioned elements in the page to show ads, and it's VERY annoying. I think we could have a -moz-z-index-threshold property in Gecko and somewhere in the popup blocker preferences a slider so the user can tweak that threshold. Z-indexes over the threshold would be out of the document acceptable z-index range and then not rendered. Most positioned ads adopt a very very VERY high z-index to make sure they'll never mess up with the page's content. I think it could work.