The Firefox browser is set to be removed from the Fire TV devices & Echo Shows by Mozilla on April 30, 2021. So if you ask Alexa to open Firefox from that point on you will be redirected to the Silk Browser instead. But on the Fire TV, you will still have access to the browser until you unstalled it from your devices. Mozilla says it’s ending support because consumers have more options for accessing content on these devices. they are shifting focus and resources to building new experiences that bring value to people’s lives and continue to offer people alternative choices.

Chard View
I personally will not miss it because the browsers on the Echo Shows are not stable, they crashed a lot & this has also been reported by my YouTube subscribers.  On the Fire TV, it is not easy to navigate web pages with the remote.