如何让FF在onkeypress处理程序添加并聚焦的输入中插入字符？

var inputElement = document.getElementById("myInput");

inputElement.addEventListener("keypress", onKeyPressHandler);

function onKeyPressHandler(event) {
  var char = String.fromCharCode(event.keyCode);
  var startPos = inputElement.selectionStart;
  var endPos = inputElement.selectionEnd;
  var inputValue = inputElement.value;
  var newValue = inputValue.substring(0, startPos) + char + inputValue.substring(endPos);
  inputElement.value = newValue;
  
  // 聚焦输入元素
  inputElement.focus();
}