This is nothing new. It has been happening since Firefox first added their "in-browser" PDF renderer. The in browser renderer does not handle the full gamut of the PDF spec (the actual PDF spec. doc. is on the order of 800+ pages long) so whether a PDF renders, or not, depends upon what portions of the PDF spec. it utilizes.
This is what the warning "this doc may not be displayed correctly" really means. It is saying: "this PDF uses PDF features we do not yet support" (but without being so explicitly direct about the meaning).
But barring a recent bug that has broken everything, the cause is inside the particular PDF's because they use features that FF's render library does not yet support.
This is what the warning "this doc may not be displayed correctly" really means. It is saying: "this PDF uses PDF features we do not yet support" (but without being so explicitly direct about the meaning).
But barring a recent bug that has broken everything, the cause is inside the particular PDF's because they use features that FF's render library does not yet support.