As we have mentioned in the earlier post, the main feature of Firefox 19 is it’s built-in PDF viewer. But if you still prefer to use a third-party tool for this then you can easily turn off the built-in viewer.
If you wish to change the default PDF viewer, you can do it by going to Options -> Applications tab. There search for PDF and select the required option from the list.
Or else you can completely disable the built in PDF viewer by going to about:config. Type about:config in address bar and press enter. Click on I’ll be careful, I promise. In that page search for pdfjs.disabled. Set its value to true and you are done.