Unpopular opinion, perhaps - isn't this just tech looking for a problem to solve?
The fact that the apps I would actually care about having on real estate on my front room seem to be nowhere to be seen is kinda glaring from my perspective... I know there's a certain purism to having Firefox front and center... But really?
I'd argue that that's orthogonal; this is making the general UI work well on a TV, then apps get populated into that. And honestly I'll bet that for most uses Firefox working well is what you want, since a handful of shortcuts to open Firefox to assorted streaming services would do the trick quite nicely.
I immediately envisioned having my feed reader as a FireFox shortcut and being able to navigate to YouTube videos from that.
Another thing is that I've found certain old movies can only be streamed from weird websites - as in https://weirdstreamingwebsite is the only licensed entity to have it. I could either buy a VHS (which is what we did) or stream it. But since a physical copy might not be available, the only option would be to use that weird website to stream it. There is no app!
Firefox, and airmouse, and an adblocker are the main tools to free yourself from commercial streaming services.