Are you sure you are using the Adobe Reader plugin?
I installed Firefox 19 yesterday, and since then it is something called PDF.js opening PDFs in the browser. So far I haven't found how to disable that (many PDF docs do not display correctly, e.g. http://helpx.adobe.com/pdf/adobe_reader_reference.pdf).
I have found how to disable PDF.jp (about:config | pdfjs.disable = True), but so far I haven't found how to re-enable the Adobe Reader plugin. Repairing Adobe Reader did not help.
I will have to do more research, but not today. Maybe someone else has some advice?