Security Researcher Matt McCutchen reported that a clickjacking attack using the certificate warning page. A man-in-the-middle (MITM) attacker can use an iframe to display its own certificate error warning page (about:certerror) with the "Add Exception" button of a real warning page from a malicious site. This can mislead users to adding a certificate exception for a different site than the perceived one. This can lead to compromised communications with the user perceived site through the MITM attack once the certificate exception has been added. 

Reference:
http://www.mozilla.org/security/announce/2012/mfsa2012-54.html

Acknowledgements:

Red Hat would like to thank the Mozilla project for reporting this issue. Upstream acknowledges Security Researcher Matt McCutchen for reporting this issue.