Test with a gov. signed pdf, other tools show the sig correctly, firefox nothing. (Address bar shows a shield that it is not tracking for not signed too. The menu has a pdf details which show nothing relevant.)
But proably you are "right". Https became a thing because big companies needed the online thing, and it has to be secure. Pdf is one of last remnant of an era, called offline, so it is an enemy. And Mozilla is not an independet thing.
Outside of the HN bubble, people are still happily scribbling ink on paper and calling it legally binding, blissfully unaware of the existence of a mechanism to mathematically prove that a specific person/key was responsible for that particular signature.
Even as someone who is aware of that, I've never had to cryptographically sign a PDF. Ever. I've either dropped a scanned image of my handwritten signature onto a PDF and re-exported it or in rare/annoying cases where it's "required", printed out the document, physically signed it with a pen, scanned it, and passed it along to whoever is making me jump through those hoops.