For some reason when downloading audiobooks from Audible, the download window in Firefox is categorizing the files as mpeg4 (video) files using the .mp4 file extension. However, unlike audio .mp4 files, Firefox is encoding them as VIDEO .mp4 files. The files do not open in iTunes nor media player as (obviously) they aren't actually video files. The files download properly as .aax files in Safari and Chrome so this is a Firefox issue and a recent one at that; thus, it could be related to the recent Version 93 update. There are a few others who have mentioned this issue in the community forum, but as far as I can tell no one at Mozilla has actually addressed the issue or offered help. Yes, you can change the file extension to .aax once downloaded and it opens fine. The point is that there is no discernible reason the files should be categorized as video files in the first place, and since this function has been working for years until recently this seems like a bug the Mozilla team should fix.