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/*.