Certificate path
If you look at the certificate presented by billing.microsoft.com in IE, it shows the certification path (the chain of trust that SSL relies on) going back to the GTE CyberTrust Global Root certificate. All good. If you look at the certificate in Firefox, the certification path is just billing.microsoft.com so it just seems to be a self signed certificate with no chain of trust.
On that basis, Firefox is quite right to flag it up, but the question remains whether it's Microsoft's certificate issuing or Firefox's certificate reading which is at fault.
Biting the hand that feeds IT
