pdf pages show black

Firefox can only present what it receives. pdfs are images. It cannot correct pdf errors and there are many broken/garbage pdfs out on the net. You need to refer to the producer of the pdf. - you may check and see if you had a bad download of the file, that happens.

I get the same issue - black pages. mostly from docs downloaded from Google Docs, but also a google doc downloaded to OPenOffice then exported to PDF.

I have the same issue, when i open them in other programs they load correctly, but Firefox wont load them. It does show a tiny version of each page correctly when i open the contents tab on the left, and a screenshot i took through Firefox's right click menu shows the page correctly