I work for US Federal courts, I can assure you html isn’t sufficient over PDF’s for court cases. Evidence are filed in pdfs. Documents (PDFs) need to be a historical archive, and the ability to modify would damage the credibility of those documents.
How are PDFs any less modifiable than HTML other than requiring (widely available) specialized tools instead of a text editor?
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.