See: *https://support.mozilla.org/kb/view-pdf-files-firefox-without-downloading-them Current Firefox versions come with a built-in PDF Viewer that doesn't have all features that other PDF readers like the Adobe Reader have or may not function properly otherwise. 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". *http://kb.mozillazine.org/File_types_and_download_actions 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).