https://blog.mozilla.com/security/2011/08/29/fraudulent-google-com-certificate/Issue
Mozilla was informed today about the issuance of at least one fraudulent SSL certificate for public websites belonging to Google, Inc. This is not a Firefox-specific issue, and the certificate has now been revoked by its issuer, DigiNotar. This should protect most users.
Impact to users
Users on a compromised network could be directed to sites using a fraudulent certificate and mistake them for the legitimate sites. This could deceive them into revealing personal information such as usernames and passwords. It may also deceive users into downloading malware if they believe it’s coming from a trusted site. We have received reports of these certificates being used in the wild.
Status
Because the extent of the mis-issuance is not clear, we are releasing new versions of Firefox for desktop (3.6.21, 6.0.1, 7, 8, and 9) and mobile (6.0.1, 7, 8, and 9), Thunderbird (3.1.13, and 6.0.1) and SeaMonkey (2.3.2) shortly that will revoke trust in the DigiNotar root and protect users from this attack. We encourage all users to keep their software up-to-date by regularly applying security updates. Users can also manually disable the DigiNotar root through the Firefox preferences.
Deleting the DigiNotar CA certificate