FireStick is a wifi device. Even if not functioning as AP but as station, it emites wifi signals.
We are used to fight weak wifi signal problems, much less concerned about too strong wifi signals.
But too strong can be a major problem (I had visiting GL.inet travel router killing my infrastructure AP's because they where too close.) My Mikrotik AP does measure the received signal and when above -30dB (eg -28dB) the receiver of the AP was saturated, even if the other device was on a different channel in that 2.4 or 5 GHz band. (Saturation of receiver happens before the channel separation, once saturation happens all received signal is distorted and invalid . Wifi is two-way communication so that saturated receiver will block sent and received information)
Set two wifi devices only 30 cm apart , emitting at the allowed +20dBm and they will saturate each other! Wifi stops functioning ! AP beacons are totally distorted and not recognised anymore. AP SSID disappears from the list. (They only disappear after a timeout, because the client caches that information for a while)
If you have a smartphone or PC you have all the instruments needed to measure. Use some APP on the smartphone that shows wifi around (Wifi Analyzer on Android, InSSIDer, WI-FI scanner, or NetSpot on PC). Interpretation is not that simple because you need to know what the FireStick and your Mango is receiving, not your measuring device. (Mango should however show signal strength in LUCI or SSH interface). Anything stronger than -30dBm is a major problem. Mango (OpenWRT) will only report on the strength of the channel connected to, another channel could be the cause of the problem.
Signal strength changes with the distance, double distance is 6dBm less (4x weaker signal), half the distance is 6dBm more (4x stronger). Good signal is between -40dBm (maximum) and -80dBm (minimum).
A flatscreen TV can reflect the wifi signal, doubling the received signal (+3dBm) if near the devices.
If the FireStick is a +20dBm device, what I expect, then 1 meter apart will be needed, as minimum.
I see the same issue happening with Chromecast devices, and the router behind the TV set.