hi, are you using a particular security software on your mac?

Just the item(s) built-in to Mac OS. As I said, Amazon Video worked previously with FF and works with my other browsers without a problem, so I doubt it is caused general security software.

firefox needs to download drm components to be able to play back drm-crippled content on platforms like amazon, because they are not directly bundled with the browser: Watch DRM content on Firefox

one common reason why this might fail is when you are not directly directed to the internet, but something on your system or network is intercepting secure connections (this is a common issue with software like avast, kaspersky, bitdefender and others...). another reason why this might not work is when you have disabled automatic updates within the firefox settings...

I have it set as always: Check for updates, but let me choose whether to install them.

I also have Content: Play DRM Content activated.