NotReadableError is the spec compliant error thrown by Firefox when webcam access is allowed but not possible.
Most commonly this happens on Windows because the webcam is already in use by another app. Firefox will throw this error on both Windows and Mac even though only on Windows processes get exclusive access to the webcam.
The error can happen for other reasons:
Although the user granted permission to use the matching devices, a hardware error occurred at the operating system, browser, or Web page level which prevented access to the device.
Chrome throws TrackStartError instead. It also throws it for other reasons. Chrome tabs can share the same device.
Source: common getUserMedia() errors .