Diagnosis
This is typically a failing T-Con (Timing Control) board or a bad connection between the T-Con and the LCD panel. It can also be a sign of a failing main board sending a corrupted signal.
How to Fix It
Power cycle the TV: unplug it from the wall for 2 full minutes, then plug it back in. Check all cable connections to external devices. Try different HDMI ports and input sources to rule out a problem with a single port or device.
If That Didn't Work
If the lines are present on the Fire TV home screen (not just on an external input), it's an internal TV fault. A repair tech would need to diagnose whether it's the T-Con board, the main board, or the panel itself.
Heads Up
Gently pressing on the bezel or frame near the lines sometimes temporarily changes them, which points to a bad connection inside the panel assembly—often not repairable.