The window.opener.closed property is supposed to return "true" or "false". However, in Mozilla and Opera if the parent window has been closed, "window.opener.closed" causes an error instead.Additionally, in IE 5.0 window.opener.closed returns "false" when the window has been closed and "true" when the window is opened.
This page illustrates the window.opener.closed problem and solution among different browsers. Note that this solution does not solve the IE 5.0 problem, ONLY the Mozilla/Opera issue. See this blog post for more information.
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