Pop Unders

Can anybody please advise how to stop Pop Unders.

I have checked the box on Firefox Options to stop Pop Ups, but I still get Pop Unders on certain sites.

The problem seems to be linked with tabbed browsing where if the site has a Pop Up it goes behind the main browser window and I only see it when I log off Firefox.

This is particularly worrying for WOT warning panels. These are to warn against bad sites and if they are hidden behind the main window for a tabbed item, it is not much use, since I enter the site before seeing the warning panel.