I agree it sounds like an HDCP problem. The general rule is to turn off everything connected to the TV, including unplugging both the TV and the Fire TV box, then restart them from the far end in - i.e. power on the FTV first, then any switches, amps, or other devices you may have between it and the TV, then finally the TV. That forces a new HDCP handshake and at least in some cases solves problems like this.
To be totally honest it started after I inserted a microSDCard into the firetv, while it was on. I didn't think then, or now, that doing so was a big deal. There is no difference in when the card is present, vs when it is out.
There wasn't an OS or firmware update to the FireTV box or TV in the timeframe of just before this started.
It's a 7 year old Vizio, some would say it's overdue and I got my moneys worth, LOL.
I plugged in my MacBook pro to the TV last night via HDMI and the "no signal" was displayed just as it is when turning on the FTV box.
Of course, that rules out the fire tv being the issue.
So long story short, I will keep this FTV and TV paired together, and will buy new FTV and new TV together in the next 30-45 days. Maybe black Friday, LOL.
Thanks for the input!!!
Oh wait, I bought a HDMI splitter to see if that improves the situation at all. I will come back and update this thread when known.