Mozilla Firefox is an open source web browser. XULRunner provides the XUL Runtime environment for Mozilla Firefox. It was found that a Certificate Authority issued a fraudulent HTTPS certificate. This update renders any HTTPS certificates signed by that CA as untrusted, except for a select few. The now untrusted certificates that were issued before July 1, 2011 can be manually re-enabled and used again at your own risk in Firefox; however, affected certificates issued after this date cannot be re-enabled or used. All Firefox users should upgrade to these updated packages, which contain a backported patch. After installing the update, Firefox must be restarted for the changes to take effect.