The support for the non-standard,
moz-prefixed values returned by the
XMLHttpRequest.prototype.responseType property, notably
moz-chunked-text, has been removed with Firefox 58 after Telemetry proved the low usage rates.
moz-chunked-arraybuffer is still available at the time of this writing, but web developers are discouraged from using the non-standard type.
moz-chunked-arraybuffer has been removed with Firefox 68.