In the most recent releases of Mozilla Firefox and Google Chrome, iframe tabs that use HTTP links will appear blank.
This is caused by a security update to the browser that will automatically hide iframes that use HTTP if you are browsing a site on an HTTPS connection.
For more information about this feature, please click here.
Facebook recommends that all content in custom tabs be secured to prevent issues with secure browsing, so all URLs entered in an iframe tab should be HTTPS. PLEASE NOTE: a website must have the proper security setup to use HTTPS, and just changing the text to HTTPS will not correct the issue.
How to set your browser to load HTTP content
To view a tab that has an HTTP iframe, click the little shield button that appears to the right or left of the address bar.
We've updated iframe tabs to now provide a link directly on the tab that users can click to view content by being redirected our of Facebook if using an HTTP URL. The editor also now better explains that HTTPS should be used if possible.
To update your existing tabs with this new feature, just republish your tabs.