"Google" is a search engine (website) and NOT (repeat: NOT) a PDF viewer. It can't and won't open PDFs in any real or imaginable way.
Firefox added their own proprietary PDF viewer with version 20 - they're on 21 now. I'll be the first to admit "it sucks". You CAN'T fill in PDF forms with it, and you can't email them from the page like with Opera, Safari or Chrome (which use the reader plug-in still).
Unfortunately, the ONLY option you have now that they've added it and killed the ability for the Reader plug-in to work with Firefox, is Tools>Options>Applications - select Portable Document Format(PDF) and choose "Use Adobe Reader (Default)". What that will do is FORCE Firefox to download the PDF instead of opening it in their terrible PDF viewer in the browser window.
Or... use one of the other browsers I mentioned. They're all free.