According to Mozilla’s research director, Dr. Andreas Gal, the idea is to use a plugin native code in the browser, either with their own Adobe Reader or other open source alternatives. With regard to safety and comparison of Google Chrome, the method to display a PDF bypasses the sandbox to prevent code injection attacks. Plugin pdf.js is based on HTML5, so it would be immune to these problems.
The extension to open the PDF in a new tab and the picture you can see a small toolbar at the top. Among the functions are integrated navigation through the document pages, adjust the zoom lens as well as print, download or add bookmarks in the PDF file.
The viewfinder found in the menu of Firefox 14 extensions but is disabled by default. If someone wants to use it, you have to disable or uninstall all plugins like PDF. The pdf.js extension can be used in the most recent versions of Firefox and is available for download from the Mozilla website.