Mozilla ended its support for Firefox on Amazon Fire TV and Echo Show devices on April 30, 2021.
This means that the app will no longer receive security updates or be available for download from the Amazon Appstore.

Since April 30, 2021, we no longer support Firefox on Amazon Fire TV or Echo Show. You are no longer able to install the app on FireTV or receive security updates starting since April 30, 2021. If you have set Firefox as your default browser on Echo Show, you are redirected to Amazon Silk for web browsing since April 30, 2021.

Users who have Firefox as their default browser on Echo Show will be redirected to Amazon Silk instead.