Not even needed, just open the Preferences, select the Applications page and for PDF instead of using "Firefox preview" select any other application, or just "Ask each time". I use this to open PDF with an external PDF reader on Linux and Windows, works like a charm.
Indeed, set pdfjs.disabled to true. I always do it, because I don't use pdfjs, but there was a pdfjs vulnerability that exposed local files to third-party JS -- so you might as well disable complex stuff that you don't really use.