After you have upgraded to Firefox 19 beta the next time you click on a link to a PDF you will find that it automatically displays the PDF in the browser using the new built-in PDF viewer.
The features are limited right now:
- View PDF files
- Select text
- Bookmark / page thumbnail navigation
The addition of a native PDF reader to Firefox comes a couple of years after Google added the same PDF viewing functionality to their Chrome browser. Up until now Firefox users had to install add-ons done by third-parties if they wanted PDF viewing functionality inside the browser.
PDF.js is a relatively new project so there are some issues rendering certain types of PDFs — in one test I did it had trouble rendering a PDF that used transparency groups — but these issues should be ironed out over time. It can take a number of years before a PDF viewer reaches maturity.
A tip of the hat to the Mozilla team for making it easier for everyone to view PDF files easily.