He probably is using an older browser without support for Server Name Indication.
$ host blog.mozilla.com
blog.mozilla.com is an alias for blog.mozilla.org.
blog.mozilla.org has address 188.8.131.52
blog.mozilla.org has IPv6 address 2620:101:8008:5::2:5
$ openssl s_client -showcerts -connect 184.108.40.206:443
subject=/serialNumber=PJYd6s/lzd2zfglc6EAG5C/hVZfSySVY/C=US/ST=California/L=Mountain View/O=Mozilla Corporation/OU=IT/CN=blog.mozilla.com
issuer=/C=US/O=GeoTrust, Inc./CN=GeoTrust SSL CA
With the latest Firefox, I get the certificate for CN=blog.mozilla.org, instead of the one for CN=blog.mozilla.com. This probably means the server is using SNI to select the correct certificate, and since his older browser did not support it, the server did not know which certificate to use and sent the wrong one.