I get a message that says, "Your web browser does not support Brotli, and therefore is not supported. Report this as a bug to your web browser vendor." I am us

I get a message that says, "Your web browser does not support Brotli, and therefore is not supported. Report this as a bug to your web browser vendor." I am using Firefox 53.02 on Linux, and my browser is sending this request header: Accept-Encoding: gzip, deflate, br As far as I know Firefox fully supports brotli compression. Any idea what is wrong?