Opening the latest version of Firefox for Fire TV now displays a warning banner at the top informing you that the app will no longer be supported after April 30, 2021. The banner remains present even while browsing web pages.
I asked Mozilla about the banner and they told me that it is permanent and that there is no way for users to dismiss it. The only time the banner is not present is when watching a video in full-screen. That means, if you want to continue using Firefox for Fire TV without a warning banner at the top, you’ll need to uninstall the new 4.8 version and sideload the older 4.7.1 version.

There don’t seem to be any other noticeable updates between the two versions, but there are definitely more changes behind the scenes than just the banner being added. The Github source has had hardly anything changed since the 4.7.1 version, but it doesn’t show any reference to this 4.8 version so it’s hard to say if everything in 4.8 is on Github. I decompiled the two APKs and there are about 100 files that differ between the two versions, so it’s definitely more than just an added banner.

Ref: https://github.com/mozilla-mobile/firefox-tv/commits/master

Users should consider using the Silk Browser on your Fire TV instead because Firefox will continue to become more and more vulnerable to attack as time passes, since it is no longer being updated.