Their root certificate is currently not trusted by any browser vendor. They have a cross-sign from IdenTrust, which is trusted even by XP. However, their interm

Their root certificate is currently not trusted by any browser vendor. They have a cross-sign from IdenTrust, which is trusted even by XP. However, their intermediate certificate uses a name constraint in a way that causes schannel (Microsoft's TLS stack) on XP to think they're not allowed to issue certificates for any domain name. Chrome and IE use schannel, while Firefox ships its own TLS stack. This bug was fixed in newer versions of Windows. This might get fixed if someone finds a way to generate an intermediate cert that doesn't trigger this bug (while still including the *.mil constraint).