Mozilla has learned that a fraudulent SSL certificate for websites belonging to Google has been issued by DigiNotar. This is not a Firefox-specific issue and the certificate has now been canceled which should protect most people.
Mozilla has released an update to Firefox to further protect you. Please update as soon as possible by following the steps in the Updating Firefox article.
You can manually delete this certificate from any version of Firefox with these steps:
Click the menu button
and choose Options.Preferences.
Click on the Advanced panel
Select the Encryption tab
Click View Certificates
In the Certificate Manager window, select the Authorities tab
Scroll down to DigiNotar and select the DigiNotar Root CA
Click Delete or Distrust...
Click OK to confirm
Because the certificate is "built-in" it will be distrusted but not deleted. Distrusting the certificate has the same effect as deleting it.