Mac OS X only: Firefox extension Quartz PDF enables inline viewing of PDFs in Firefox 3 for the Mac. Just install, and next time you follow a link to a PDF, it quickly loads the PDF directly inline. Quartz PDF is lightning fast, and works exactly like you're viewing the PDF in Preview. It'd be great to see the thumbnail navigation sidebar in a future release, but as is this extension works like a charm. Quartz PDF is free, requires Firefox 3 and OS X 10.4 or higher.