Got problem with this in Firefox 3.5?
To protect your security, the publisher of this content does not allow it to be displayed in a frame
If you have NoScript installed, that’s your problem. Either disable NoScript, or go to about:config and add your site to option noscript.frameOptions.parentWhitelist — for example, the default over there included https://mail.google.com/* but I needed http://mail.google.com/*.
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via blog.vucica.net