How to disable the built-in PDF viewer and use another viewer
Third-party PDF viewers are available, if you prefer not to use the Firefox built-in viewer. To switch from the built-in PDF viewer to another viewer:
Click the menu button
and choose Options.Preferences.
Select the Applications panel.
Find Portable Document Format (PDF) in the list and click on it to select it.
Click on the drop-down arrow in the Action column for the above entry and select the PDF viewer you wish to use.
Note: If you want to view PDF files in Firefox using a plugin, instead of in an external PDF reader application, select Use PDF plugin name (in Firefox). For more information, see View PDF files in Firefox.
Close the about:preferences page. Any changes you've made will automatically be saved.