UPATED: Recently I started to experience network buffering issues. I performed all of the optimization techniques, but my WiFi signal was below 1 Mbps, even though I have 10 Mbps signal from my router. The problem is overheating inside the USB controller (Fire TV Basic). My simple solution was to remove the plastic cover but there are other solutions you can find online.
Upgrade path delivered Alexa voice, volume control to my soundbar and TV input switching. Great so far, but after 5 days <;Select, Navigate> cursor movement caused my Fire TV to re-boot. This makes an otherwise great product virtually useless. I contacted Amazon technical support 3 times but their suggestions were just a waste of my time; un-pair, power off, remove batteries, etc. Finally Amazon authorized a replacement remote, but after doing research on the symptoms, the real cause (and other suggestions too) solved the problems I experienced. I'm surprised with so many identical reports of this re-boot issue that their tech support is not aware of the real problem or willing to admit there is an issue.
Here's my solution. The Fire TV Basic unit came with a 1.0 A USB power supply. As I added functionality with the upgrade Alexa remote, the power consumption became greater than 1.0 A. As a temporary measure, I substituted a Blackberry 1.3 A power supply and so far no re-boots. I have ordered a 2.4A (maximum) power supply to permanently solve the problem. I speculate when the firmware encounters an error condition it cannot solve, it simply re-boots with no error message.
I hope Amazon recognizes this fault and includes a replacement USB power supply with future Alexa remotes? I also discovered that any peripheral that uses a USB connection for power ; i.e. an HDMI switcher, may discover the USB power source, possibly even a TV USB port is not powerful enough.