To change from using the built-in PDF viewer to your PDF reader plugin:
- At the top of the Firefox window, click on the Firefox button and then select Options
- Select the 'Applications' panel.
- Select 'Portable Document Format (PDF)' from the Content Type list.
- Click on the drop-down arrow in the Action column and select 'Use Adobe Acrobat (in Firefox)' or your particular PDF reader plugin (if not Adobe Acrobat).
For more information check the related Firefox support page