TECH TOLA

This articles also helps you to achieve exactly opposite of what is given in the title and use the same to control opening of other file formats like audio, video inside Firefox.

Reasons why you might want to do this

1. Firefox becomes slow and unresponsive, when huge PDF's are opened within.
   
   
2. Your intention is to download PDF only.

On the Menu bar click
Tools > Options > Select Applications tab on top, then scroll down.

Select Adobe Acrobat Document Options as shown in the picture.

The Easiest way is to just right click the PDF link and select "Save Link As".

But that doesn't work for a few websites in my experience. Moreover I wanted to help people about controlling Videos and Audio files that open inside Firefox.