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.