In older Fire TV and Fire Stick models, allowing ADB debugging (in order to sideload unofficial apps, for example) opened up the device to vulnerabilities -- you allowed an ADB connection once, so all future connections were allowed by default, including nefarious bugs. In newer models -- and with the update for older ones -- you'll be prompted to allow or deny an ADB connection every time one is requested. So if your Fire TV prompts you to allow a debugging connection you didn't initiate, say no and investigate.