Did you previously select Firefox when you got the 'open with' dialog for a PDF file? you should never select Firefox in such a case. 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+keeps+opening+many+tabs+or+windows This file is send as content-type: application/pdf and Firefox should use the builtin PDF Viewer to display this file. Is the Firefox PDF Viewer enabled ? Is "Preview in Firefox" selected for Portable Document Format (PDF) if you check this in "Options/Preferences -> General -> Applications" ? ---- *https://support.mozilla.org/en-US/kb/view-pdf-files-firefox *https://support.mozilla.org/en-US/kb/disable-built-pdf-viewer-and-use-another-viewer