Firefox and Audible.com

Hi all, For years, I have used Firefox to download audiobooks from Audible.com. I go to Audible.com, open my library, click download, and then a pop up shows the .aax file I'm going to download. The problem - In the past week, the pop up window now shows that I'm about to download a .mp4 file. That doesn't play nice with Apple Books or my iPhone. I've called Audible several times, but they haven't been able to help. If I use Safari, the .aax file is downloaded correctly. This is a Firefox issue. Any ideas here? Thanks for your time and help! --Scott