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/*.