See this support article. https://support.mozilla.org/en-US/kb/disable-built-pdf-viewer-and-use-another-viewer

You can change the action for Portable Document Format (PDF) from Preview in Firefox to use another application like the Adobe Reader or set to Always Ask in "Firefox > Options/Preferences > Applications". You can set the pdfjs.disabled pref to true on the <b>about:config</b> page to disable the build-in PDF viewer. You can check the value of the plugin.disable_full_page_plugin_for_types pref on the about:config page and remove the application/pdf part if present or reset the pref to the default via the right-click context menu if you want to display PDF documents in Firefox with another application (i.e. not the built-in PDF Viewer). See also: *https://support.mozilla.org/kb/view-pdf-files-firefox-without-downloading-them *https://support.mozilla.org/kb/disable-built-pdf-viewer-and-use-another-viewer