when I do apt-get update in Terminal I get an error saying:

GPG error: http://moff.mozilla.com chinook Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 667387BFFF445C24

What is causing this and would it be the reason then when I go do apt-get upgrade it says:

Errors where encountered while processing:
krb5-config
heimdal-clients
E: Sub-process /usr/bin/dpkg returned an error code (1)

I don't know what to make of them can anyone help?