By default the Acrobat Reader try to open remote PDF files in Firefox, which could make Firefox slow down so this is a tweak to make firefox gives you the choice between download the PDF or open it directly from internet.
To do so Go to “Tools”, then “Options”, and choose “Downloads” tab. In the “Plugins” list find PDF type and uncheck the “Enabled” option. Now everytime you click on a PDF file Firefox will ask you what to do with this file
