Change your plugin *https://support.mozilla.org/en-US/kb/view-pdf-files-firefox-without-downloading-them#w_using-a-pdf-reader-plugin

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. *http://kb.mozillazine.org/about:config See also: *https://support.mozilla.org/kb/view-pdf-files-firefox-without-downloading-them