Changes [Oct 01, 2018]Switches
If you can't open PDF's in Firefox, or you are printing blank pages when printing a PDF in Firefox, you can change how Firefox handles PDF's by changing the following settings in the Firefox browser. First you will need to open the Firefox browser and click "Tools" at the top of the window. Then click "Options".
This will open the Firefox Options window. Make sure you are in the "Applications" tab of the window, and scroll down to where it says "Portable Document Format (PDF)". Click on the drop-down box next to this entry and then click "Use Adobe Acrobat (In Firefox)". Then click the "OK" button.
That's it. You should now be able to open PDF's in Firefox and print them correctly.