Today using Firefox 93.0 on my Windows 10 home computer I opened the browser to my Audible library on Audible.com and selected a book to download. The browser download window opened with the book file name but the extension wasn't .AAX but had been changed to .MP4. This did not happen when I used Microsoft Edge nor did this happen on my Windows 10 laptop which was still running Firefox 92.0. Once Firefox upgraded to 93.0 on my laptop I now get the same problem as I get on my home computer. Audible technical support looked into the problem and found that this change wasn't caused by their servers. I have discovered that if I change the file extension from MP4 to AAX I can load the audible book into iTunes and sync it over to my iPhone which is able to play the file as an audible book. This indicates to me that the actual AAX file was not converted to MP4. For some reason only the file extension was changed. I wasn't able to find any setting in the Firefox browser that corrected this problem.