You get tabs opening endlessly if you select Firefox as the application to handle a file when you get an "Open with" dialog. *https://support.mozilla.org/en-US/kb/firefox-repeatedly-opens-empty-tabs-or-windows You need to change the action for "Portable Document Format (PDF)" to "Open in Firefox". *Options/Preferences -> General -> Applications *https://support.mozilla.org/en-US/kb/change-firefox-behavior-when-open-file