The NPAPI plugin by Adobe was blocked by FireFox because it caused the browser to crash...

You need to first disable the NPAPI plugin by going to Tools --> Add Ons and disabling that...

Then download the PDF Viewer Add-On that Firefox has for it's browser here: https://addons.mozilla.org/en-us/firefox/addon/pdfjs/

Then to make sure it's all working... Go to your preferences (command+,) and click "applications"... Make sure the default is selected "Use Adobe Acrobat Pro (default)" -- (you might have to restart after adding the add-on before it shows up, not sure.

Relevent sites:

https://addons.mozilla.org/en-US/firefox/blocked/p89

https://support.mozilla.org/en-US/kb/change-firefox-behavior-when-open-file

LOOK NO FURTHER - if you have a MAC anyway ; )

Go to your MacIntosh HD (hard drive) Double click on Library Double click on Internet Plug-ins Scroll down to AdobePDFViewer.plugin (DELETE IT - OR SEND TO TRASH) Scroll down again to AdobePDFViewrNPAPI.plugin (DELETE IT - OR SEND TO TRASH) Then Reboot your system and voila .. you're done!

In my case I solved this problem by disabling the extension of Nitro PDF download which was causing the conflict.Hope this will help others.

I only get a white page when attempting to download a pdf file.In add ons it says my plug in was automatically disabled due to a plug in vulnerability although it appears to be enabled.I tried disabling it,nothing.I clicked on the update button.It downloaded the update and failed to install due to some bootstrap problem.Tried uninstalling adobe and reinstalling.No help.

    Finally I went back to windows explorer and was once again able to download pdf files.
   This had happened before,not too long ago, and I had to upgrade to the newer version of adobe.This time it appears nothing can be done.Firefox updates fail.Guess I will be using explorer from now on.