pdf.js does work with TenFourFox, is a safe alternative, and is
used by some users. However, it does not support many features of
PDF files and is slower than Preview or Acrobat. It is a part of
TenFourFox 15, although it is not enabled by default.
We don't support pdf.js here; it would be better to discuss
issues with it with its programmers directly.
You probably would be better served by disabling pdf.js if it
isn't working for you. Assuming no other unusual settings on your
system, you will then be prompted to download PDFs and can open
them in Preview.app. If disabling pdf.js doesn't allow you to
download files again, you probably should start with a clean
profile to ensure a stray setting isn't interfering; see Help!
Where Do I Begin?? at the lower right.