pdf.js is an HTML5-based Portable Document Format (PDF) renderer.
The pdf.js project is community-driven and supported by Mozilla Labs. Its goal is to create a general-purpose, web standards-based platform for parsing and rendering PDFs, and eventually release a PDF reader extension powered by pdf.js.
Integration with Firefox is a possibility if the experiment proves successful.
More information can be found on GitHub (including an online demo) and on the MozillaWiki PDF.js page.