You may already encountered websites that provide link to a PDF file, usually if you installed Adobe Reader you will be able to automatically view the PDF files right inside your browser.
The trouble with this method is, your browser may freeze or crash while waiting for the PDF files to download to your computer. Furthermore, some people view this feature as a drawback because Web browser should not takeover PDF reader job.
The Solution
Fear not because somebody has published a quick fix that disable firefox capability to display PDF files on the fly.
Please proceed to Disabling PDF Display in Firefox