The fact that Mozilla's well-known name was attached didn't help. That made unsuspecting users trust malicious uploads/links.
I think that's the reason, why this service is not very interesting for illegal content.
Immorality is extremely nuanced and heavily influenced by society. Eg. homosexulity is illegal in some south Asian countries such as Indonesia, but is not seen as immoral in most parts of the world.
Conflating the two - or preferring the term "immoral" over "illegal" inherently lends itself to allowing people to rationalise illegal activities - such as child pornography as only being "disapproved of by society" (morality being a fairly abstract concept) rather than an act which violates the laws of the land and is officially and formally condemned by the government.
Why do you need to rationalize illegal activities if they aren't immoral?