I ran across the Amazon FireTV "Live TV Sources" feature quite by accident, missing my intended key and pressing the button with a TV symbol instead.
My FireTV Cube came with the five sources seen in the picture below which if you're a fan of "channel surfing" you can scroll through over 600 channels looking for something new (or perhaps something old). To this list, I added Plex and Tubi then sorted the sources so the two largest, freevee and YouTubeTB, were at the bottom of the list. I'm not sure how often I might use this feature, but it's nice to have it available.
The best thing I found while exploring this feature is that "Kung Fu", the 1970s series starring David Carradine as Kwai Chang Caine is available on Plex and it's one show that I've wanted to watch again. It's the right time of year to have binge-able content.