We as a company deal with a lot of PDF files.. Now while we can simply download the PDFs and there not be a problem, we're used to the Adobe Viewer and *not* downloading them.
I've been told the new pdf.js viewer will not display most of the PDFs we throw at it on a day to day basis.
Although there is a way to disable pdf.js (about:config), this only seems to change for the local user. We have FF 19 installed on terminal servers, and would like to know if there is a way to disable it for all users?
At first I tried setting pdfjs.disabled, and like you mentioned, it would download PDFs instead of opening them in the Reader plugin.
What worked much better was clearing plugin.disable_full_page_plugin_for_types. When upgrading to FF19+, it adds "application/pdf". Obviously if there are other values in there, or other values need to be added later, locking it empty will break that. I'm trying to find out if (a) you can parse and edit a value instead of merely setting it or (b) whether there should ever be other values in there.
If you find an answer to either of those, make sure to post it here.