Popups are small windows that display automatically, and without your permission. These windows may display on top of the web page you are viewing (popups), or below the web page (pop-unders). Pop-ups are often advertising, and may be annoying or intrusive; Mozilla therefore enables you to block unwanted popups (this includes pop-unders) from displaying.
In some cases, web sites (or web-enabled applications) will use popups as part of their function. In these cases, Mozilla popup management enables you to selectively permit popups to display.
