That's really unfortunate

And a reason why I stay away from
smart TVs when possible.
The browser on a FireStick can connect to IMatch Anywhere, but may have some usability issues. Since Mozilla no longer develops for FireTV, you're stuck with the browser Amazon includes.
I have a small box PC attached to my TV. The size of a cigar box.
It has a quite fast 4-core Intel CPU, 8GB RAM and is absolutely noiseless (no fan!).
It runs Windows 10 and Linux Mint in a dual-boot configuration.
I control it with a small bluetooth keyboard with a touch panel. Works a treat and gives my TV all the smarts I'll ever need.
While being under my control and not phoning home or sending info about every movie or TV show I watch to the TV vendor and probably hundreds of other listeners.
I can run a real web browser (Firefox, Chrome, ...), have access to all standard applications, can run Kodi with my video collection, Disney, Netfilx, Prime in the browser when I need them.
And it will not stop working in two years when the warranty of the TV runs out and the TV vendor no longer cares for the "old" apps shipped two years ago. Or just switches of the servers.