File type changes by Firefox
When dowloading an AAX type file from Audible, Firefox changes it to MP4 (downloads as AAX with other browsers).
Note, changing the file type after the download fixes the problem. But, is there a way to prevent it from happening in the first place? Also, why changing file type in the first place?
Try to set browser.download.sanitize_non_media_extensions = false to see if that makes Firefox show the correct file extension when you save the file.
You can open the about:config page via the location/address bar. You can click the button to "Accept the Risk and Continue".