They used to have a far more expansive certificate, with a subject alt. name good for *.google.com, and I think google.com as well, so they could use it on subdomains like adsense.google.com. It was a really handy server for testing TLS implementations with big complex certificates. It was also sort of illegal, having both a common name and subject alt. names. However, the certificate expired and they got a new one early this year, and now it's some bog standard thwate issued cert, good only for one year (Hello! You have billions, you can afford to get a certificate issued for more than a year at a time guys!)
Anyway, you should really only be warned in the event of domain name mismatch. This isn't a fatal error. I guess firefox has gone overboard on the treating users like idiots front.
For some reason, Google has SSL certs for www.google.com, and tries to use that cert for google.com connections too.