Disable Update w/o Root Steps:
1. Enable USB Debugging on your Fire TV (or TV Stick) (how-to here)
2. Setup adbFire on your computer and establish a connection to your Fire TV[Stick] (how-to here)
3. Setup Debloater tool (how-to here)
4A. In the Debloater tool, check off the APK below then hit the "Apply" button:
- DeviceSoftwareOTA.apk (com.amazon.device.software.ota)
adb shell pm hide com.amazon.device.software.ota
After hiding the apk your Amazon Fire TV or Fire TV Stick will no longer get OTA updates.
If you go into Settings > System > About you can no longer check for updates... yay.
To undo the block you can simply "unhide" each apk using the Debloater tool or run:
adb shell pm unhide com.amazon.device.software.ota
You gotta update to Fire OS 5.x in order to block updates without root (or router).
I know this defeats the purpose of blocking updates since you want to stay on old fw/sw... but science!
This may not work on all future Fire OS 5.x+ builds. Tested on FireTV Stick with Early Developer 5.0.3 build.