In Firefox, type "about:config" (minus the quotes) in the address bar.
A window will open with a warning.
Click "It's OK, I'll be careful" and open the configuration window.
Type "js" (again, minus the quotes) in the search bar at the top.
In the results you'll see "pdfjsviewer disabled" with a value of "true".
Right click "true" and toggle it to "false".
Close the configuration window.
In Firefox options (Tools>Options for Windows - Firefox>Preferences for Mac), under the Applications tab:
Locate Portable Document Format (PDF) or Adobe Acrobat Document(PDF)
Click the drop down on the right and select Adobe Reader as the app to preview in Firefox.
This will disable Firefox's built in PDF viewer and enable Adobe reader's plugin for viewing PDFs.