I downloaded firefox 19 and executed it on my Ubuntu 12.04 machine. I went to a website with pdf links, clicked on one of them, hoping to have it shown in the built-in pdf viewer in Firefox 19, but only got the common pop-up menu with the options of viewing it with Evince or downloading it to a folder. How can I use Firefox 19 built-in pdf viewer?
Go to Edit → Preferences → Applications and select Preview in Firefox for the content-type Portable Document Format (PDF).
Also, to confirm whether pdf.js (the inbuilt pdf viewer) is enabled, type
about:config in the browser's address bar, press Enter, accept the warning, and, in the next screen, enter
pdfjs in the search bar; you should see an entry with pdfjs.disabled in the Preference Name column. If its value is true, highlight that line and press Enter to toggle and set the value to false.