Rather than having the login appear in an IFrame lightbox within the browser window, the program now opens a separate browser window, with an address bar, Naitik Shah, an engineer on the Facebook Platform team, posted on the blog.
In order to make a more consistent and secure user experience when connecting accounts, we’re changing the behavior of the dialog when a user connects accounts between your site and Facebook when the user is already logged in to Facebook. Previously, the Connect with Facebook dialog appeared in an IFrame “lightbox” within the browser window. After tomorrow’s weekly code push, the dialog will pop up in a separate browser window, with an address bar, which matches the user login flow when the user is not logged in to Facebook and tries to connect accounts with your site. This will also make it easier for users to identify that the URL is trustworthy and coming from Facebook.
If the user has blocked browser popups, Facebook displays the following dialog (You must log in to perform this action), prompting the user to log in to Facebook. The user will then be directed to facebook.com to log in, and then back to your site after logging in to connect accounts. Users can also white-list facebook.com for pop-ups.