Adobe Reader is a free program for viewing and printing PDF documents.
It includes an Adobe Acrobat browser plugin for displaying PDF documents within Firefox and other Web browsers. (By default, Firefox uses its built-in PDF Viewer.) This article provides resources for setting up Adobe Reader and the browser plugin on your computer.
To install Adobe Reader, please visit the Adobe.com website. To make sure your installation of Adobe Reader includes the latest available security and stability updates, open Adobe Reader, click on the Help menu and then click Check for Updates….
Click the following link to test how PDF files are displayed in Firefox:
By default, Firefox will use its built-in PDF Viewer. To use the Adobe Reader program or browser plugin instead, open the Firefox Applications panel, select the Portable Document Format (PDF) entry and change the action to use either the Adobe Reader program or the Adobe Acrobat plugin. For more information, see How to disable the built-in PDF viewer and use another viewer and View PDF files Firefox - Using another PDF viewer.
Based on information from Adobe Reader (mozillaZine KB)