Try to rename the cert8.db file in the Firefox profile folder to cert8.db.old or delete the cert8.db file to remove intermediate certificates that Firefox has stored.
If that helped to solve the problem then you can remove the renamed cert8.db.old file.
Otherwise you can rename (or copy) the cert8.db.old file to cert8.db to restore the previous intermediate certificates.
Firefox will automatically store intermediate certificates when you visit websites that send such a certificate.
You can use this button to go to the Firefox profile folder:
Help > Troubleshooting Information > Profile Directory: Show Folder
Re: Can't load www.google.com uses an invalid security certificate. The certificate is only valid for the following names: see below in details
- Make sure that the date and time on your PC are correct.
- Sites have certificates for periods of time (01/03/2012 - 01/03/2013 for example).
- If the date and time on your PC are not correct then your browser might think that the site you are trying to view has an expired certificate.