How to turn off safe mode on Fire TV cube 3rd Gen?
1. Fire TV Cube 3rd Gen supports Safe Mode. To boot into Safe Mode on the Fire TV Cube, disconnect the power cable, press and hold the volume down button on the top of the Fire TV Cube, and then connect the power cable.
2. Continue to hold the Volume Down button until you see "Safe Mode" appear in the lower left corner. It should appear immediately after the "Fire TV" boot animation disappears.
3. To exit Safe Mode, simply restart the Fire TV Cube by going to Settings > Devices > Restart
In Safe Mode, no 3rd party applications will be able to start. Only system apps that were installed on the Fire TV Cube at the factory can be launched. You can stream video from Amazon in safe mode, but not from any other streaming service or app.