firefox 19 pdf viewer for ubuntu?

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http://askubuntu.com – 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? (HowTos)