I see those from time to time, particularly on travel-related sites where they want one last interaction after you close their page. I believe pop-unders are handled using the same rules as pop-ups, as they uses the same method to open (the difference being which window ends up having the focus). The problem is that some pop-ups need to be allowed for functionality reasons. The general principle used in Firefox is that they can be opened followed a user-generated event, such as a click. Sites often find ways to abuse this. You can find the list of "events allowed to trigger popups" listed here: (1) In a new tab, type or paste '''about:config''' in the address bar and press Enter/Return. Click the button promising to be careful or accepting the risk. (2) In the search box above the list, type or paste '''pop''' and pause while the list is filtered (3) The '''dom.popup_allowed_events''' preference lists the allowed hooks that do not get filtered by the pop-up blocker (I have grouped them for discussion): * Mouse or touchpad events: click dblclick mouseup pointerup notificationclick * Touchscreen events: touchend * Form-related events: change reset submit You could experiment by removing some of these from the list and seeing which changes are effective in tests of known popunder sites. This will help in recognizing which events are being hooked to show those new windows. If something important breaks (assuming you recognize the issue), you can restore the list using right-click > Reset.