Yeah, but does Firefox need to solve the use-case of a court system? Also, tangentially the solution to guarantee "tamperproof" archiving is in cryptography and that's not a feature of PDF.
Also, cryptographic signatures do happen to be a feature of PDF.
What I meant to say is that Firefox should focus on implementing cryptographic signing over HTML then. And not a PDF viewer on the web--in that, enabling cryptographic signatures isn't tied to the format PDF per se.