Firefox cannot natively open PDFs, and the Adobe plugin doesn’t work in Forefox under OS X. Schubert IT has created a PDF Browser Plugin that I’ve found to be very useful. Just download the disk image, mount it, and move the PDF Browser Plugin file to /Library/Internet Plug-Ins (or ~/Library/Internet Plug-Ins to install this only for your account). You should note that installing this plugin will cause it to be used by some other browsers, including Safari and Opera. Acrobat and Quicktime can override this behavior - details are in the Read Me file on the PDF Browser Plugin disk image.