I purchase audiobooks on Audible.com using my Mac (OS Catalina). When I download a purchase Firefox sends it as an MP4 file which is incompatible with the apps to play the audiobook. Safari and other browsers do no do this - the files come as an .aax file. Even after I have an .aax file I cannot play it using Firefox because Audible.com does not recognize my registered computer. I get an error message: Document "LicenseForCustomerToken" couldn't be opened. Incompatable with QuickTime Player. FYI: QuickTime Player is a video (MP4) player app on my computer. So, once again, Firefox makes the .aax file appear like an MP4 file to Audible. It's doing some kind of unrequested and unwanted conversion of .aax files to MP4. I hope you can fix this in your next update. i've worked with Audible to solve this issue, but they cannot help and are telling me it is a "known issue" with Firefox and recommend I use a different browser.