If you browse using Firefox and use the Google Reader keyboard v shortcut that I wrote in an earlier post, you may find that Firefox will eventually generate this message,
Firefox prevented this site from opening a pop-up window, although you have added www.google.com to the Allowed Sites - Pop-ups dialog. This behaviour occurs because Firefox prevents any site from automatically opening too many popups.
According to Lifehacker's Increase Firefox's Maximum Pop-up Count, the number of pop-up windows a site is allowed to show is controlled by the dom.popup_maximum variable. By default, it is set to 20, so to avoid hitting the limit, I increased this value. Of course, now my browser is more vulnerable to a pop-up window attack or runaway script from all sites listed in the Allowed Sites.
Perhaps a better solution is to have a per-site limit?