The following article details this (https://support.mozilla.org/en-US/kb/disable-built-pdf-viewer-and-use-another-viewer):

To switch from the built-in PDF viewer to another PDF viewer:

  1. Click the menu button New Fx Menu and choose Options

  2. Select the Applications panel.
  3. Find Portable Document Format (PDF) in the list and click on it to select it.
  4. Click on the drop-down arrow in the Action column for the above entry and select the PDF viewer you wish to use.

    PDFviewer&plugin-Fx19
    Note: If you want to view PDF files in Firefox using a plugin, instead of in an external PDF reader application, select Use PDF reader name (in Firefox).
  5. Click OK to close the Options window