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
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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.
OK, so you want to stamp your document. Maybe you need to give reviewers some advice about the document's status or sensitivity. This tip from author Ted Padova demonstrates how to add stamps with the Stamp Tool along with related comments.