Firefox does not open PDF files in browser. You would need to use a plug-in such as pdf viewer to be able to read PDFs in browser. Some of these plug-ins may be buggy and spammy however. Alternatively, you can save, download and open the PDF from the link in the browser to your hard drive and then open. Otherwise, it is recommended that if you really want to read PDF files in browser, to use Safari or Chrome, both of which do open PDFs in browser.
Link to the discussion article: