jsignpdf seems to be updated on 2019-04 and it worked fine for me! Thanks for all the info in this thread.
I haven’t been able to get my LibreOffice Draw 184.108.40.206 to work since a while. It always displays the “This document has an invalid signature” notice when I open signable PDFs with it, and when I click on Digital Signatures, nothing pops up. I already checked that the certificates folder points to my Firefox profile and that a personal certificate is added there. Any clues on why that may be?