When closing after a browsing session I find several windows behind the main browser window that I haven't opened and they must be closed individually. What is causing this and how can I stop it?

Do these windows display ads? They might be "pop-unders" that are designed to show when you finish with your main window. Orbitz has these, for example. Irritating. I'm not sure of the best solution, i.e., pop-up blocker vs. ad blocker. If they aren't ads, can you think of any way they are related to the sites you're visiting? Also, to close a tab (or a window, if there's only one tab), you can press Ctrl+w. Might speed up the process of clearing them until you find a real solution.