You need to change a setting in Firefox to disable built-in PDF viewer. In the address bar type about:config and press enter. Accept the warning message that comes and then you will see the list of settings that can be changed. In the search bar of the page, type pdf and you will get access to the setting you need to change, as shown in picture bellow.
You will see a setting ”pdfjs.disabled” with a value as ”false”. Right click on the setting and click on ”toggle”. The value of the setting will now get changed to ”true”, as shown in picture bellow.
From now your Pdf files inside Firefox browser will use your default PDF viewer to view PDF files instead of the built-in viewer.