If the server does not force a download, or if you use an extension that overrides Content-Disposition from attachment to inline, Firefox will happily open MP3 and MP4 files in their own tab using the "stand alone" media player. There is a preference to disable the stand-alone media player, which you can toggle from true to false as follows: (1) In a new tab, type or paste '''about:config''' in the address bar and press Enter/Return. Click the button accepting the risk. (2) In the search box in the page, type or paste '''media.play-stand-alone''' and pause while the list is filtered (3) Double-click the preference to switch the value from true to false