Experimental technology likely not handled correctly by Firefox.
Try a `SpeechSynthesis.cancel()` before re-iterating your `.speak()` call.
<!-- begin snippet: js hide: false console: true babel: false -->
<!-- language: lang-js -->
var synth = window.speechSynthesis;
let msg = new SpeechSynthesisUtterance("say something");
synth.cancel();
msg = new SpeechSynthesisUtterance("say something");
synth.speak(msg);
<!-- end snippet -->
