|Invalid Certificate Message|
The way I solved this problem was first to revoke the certificate that had been assigned to me. Next, I started the process again, this time using Internet Explorer. So I started IE, and then went to Comodo and signed up for the certificate, and then I claimed and installed the certificate using Internet Explorer.
When I went into Outlook 2013 and signed a message, everything worked!
I don't know why this works, it just does. Perhaps Firefox uses a different location to store these certificates and it freaks Outlook. Give it a try and let me know if this works for you.