See PDF files are blank or cause an error message on Mac

Your plugins list shows Adobe® Acrobat® Plug-in for Web Browsers. Try setting another PDF viewer for Portable Document Format (PDF) in Firefox Preferences - Applications panel.

I followed these directions but still no go. PDFs still do not open in Firefox. I have disabled the Adobe Acrobat Plug-In and tried both Preview and the Ffx default PDF viewer, and still nothings works.

What happens when you try to open this PDF file? (It's linked from the Use Adobe Reader to view PDF files in Firefox article.)

http://static.mozilla.com/moco/en-US/pdf/mozilla_privacypolicy.pdf

If that PDF file doesn't open then you could try to restart in Firefox Safe Mode. Details on that here: Diagnose Firefox issues using Troubleshoot Mode Do PDF files open in Firefox Safe Mode?

(You can also try a Refresh but see this article first: Refresh Firefox - reset add-ons and settings