Do you have any problems there? If not, don't worry about it. Some web sites do not like Firefox.

I have the same issue, but not with amazon. It says:

"For more features and less buffering, we recommend using the HTML5 video player on Google Chrome, Internet Explorer 11 on Windows 8.1 or later, Microsoft Edge, or Opera."

I don't know who the "We" is. It occurs in latest Firefox 42 under Windows.