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