URL: https://goldacct.co/firefoxerror.htm

Browser / Version: Firefox 65.0
Operating System: Ubuntu
Tested Another Browser: Yes

Problem type: Desktop site instead of mobile site
Description: Speech Synthesis play incorrect
Steps to Reproduce:
I wrote a simple javascript as below.
when I set lang = 'cmn' (Mandarin Chinese).
it works well in firefox for Android, or Chrome mobile and desktop versions.
however, in firefox desktop version for Ubuntu 18, the speech is unrecognizable.

=========== java script Begin =================
function Sayit() {
let u = new SpeechSynthesisUtterance();
u.lang = 'cmn';
u.text = "早安";
speechSynthesis.speak(u);
}
=========== java script End =================

Reported by @stevene29f

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