Firefox 3 browser does not have any feature to view PDF files in it. In fact it downloads the PDF files and opens them in your Adobe Reader (Foxit Reader recommended), which is a double pain for most of us.
To directly view them in your browser, I use a Firefox extension called PDF Download. Using this addon, you can do whatever you like with the PDF files online. PDF Download by Nitro PDF Software is the leading tool for handling and viewing Web-based PDF files and is enjoyed by millions of Firefox users every day. The browser extension removes the pain often experience when encountering PDF files online, and now lets you quickly convert your Web pages into high-quality PDFs that are ideal for archiving, printing and sharing.