How can I use Firefox's built in PDF viewer?
Go to Edit → Preferences → Applications and select Preview in Firefox for the content-type Portable Document Format (PDF).
Also, to confirm whether pdf.js (the inbuilt pdf viewer) is enabled, type about:config in the browser's address bar, press Enter, accept the warning, and, in the next screen, enter pdfjs in the search bar; you should see an entry with pdfjs.disabled in the Preference Name column. If its value is true, highlight that line and press Enter to toggle and set the value to false.