Quote
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for public websites belonging to Google, Inc. This is not a Firefox-specific issue, and the certificate has now been revoked by its issuer, DigiNotar.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.
http://blog.mozilla.com/security/2011/08/29/fraudulent-google-com-certificate/
You can also
manually disable the DigiNotar root through the Firefox preferences.
Instruction for K-Meleon users.
Tools >
Privacy >
View Data >
View Certificates > select
Authorities tab. Scroll down to DigiNotar and select the DigiNotar Root CA, click Delete, then OK to confirm the deletion.