Mozilla has been informed about the issuance of several fraudulent SSL certificates for public websites. The certificates have been revoked by their issuer which should protect most users. Current versions of Firefox are protected from this attack. The issue was reported to us by the Comodo Group, Inc., the certificate authority responsible for issuing the fraudulent certificates. We encourage all users to keep their software up to date by regularly applying security updates. We are still evaluating the possibility of further response to this issue. We urge users to regularly apply security updates to their software.”]
Source: https://blog.mozilla.org/security/2011/03/22/firefox-blocking-fraudulent-certificates/