Amazon FireTV Devices

In order to change the Plex video quality settings on your Amazon FireTV device, simply find the “Settings” menu option at the bottom of the home screen in Plex.

Settings > Video Quality >

Head down to the Video Quality section and first you'll want to select the Quality Suggestions option and toggle it OFF

This is what your final settings “should” look like…
Adjust automatically > OFF
Home streaming > Maximum
Quality Suggestions > OFF
Remote streaming > Maximum