Signing using a PKI key identity, I don't know if anyone has though of this but implementing it on your PDF viewer / editor would be game changing for a lot of agencies that use certs to sign and validate user identity.
@marcoI was thinking both. Though it is just a thought. Not super familiar with how the cert chains would verified using FireFox. Also I am by no means an expert on how any of this works but I can provide screen shots of what my thought process is with what a lot of orgs use with adobe reader.