X-Frame-Options
One of the security enhancements included with Firefox 3.6.9 is support for the x-frame-options header. This optional header can be included within the HTTP response to instruct the client’s browser on whether the returned content is allowed to be framed by other pages. A website can choose to include the x-frame-options header to protect against malicious framing of web content by third parties.
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X-Frame-Options