It gave you a link to be shared, that essentially embedded the cryptographic key to decrypt. It's basically the same scheme that e.g. mega.nz uses too (their "s

It gave you a link to be shared, that essentially embedded the cryptographic key to decrypt. It's basically the same scheme that e.g. mega.nz uses too (their "share link" feature), and similar encrypted services. These links weren't just shared one-on-one in some private channels, but people posted the links in (semi-)public places as well. If it was something actually bad or at least "bad", a lot of times other users, law enforcement, or copyright owners would report these links back to mozilla, requesting the link be disabled and the content taken down.