KeyEvent.initKeyEvent which is a legacy API will be dropped from Gecko because it's not supported by Blink/WebKit. comm-central still uses it in some places:
https://searchfox.org/comm-central/search?q=initKeyEvent&path=&case=true&regexp=false

Recommended solution is using:

let event = new KeyboardEvent(eventType, {
  bubbles: true,
  cancelable: true,
  keyCode: someting,
  charCode: anything,
};

But using the event constructor changes which value.

I'll look at this tomorrow. Thanks for letting us know.

Pushed by geoff@darktrojan.net:
https://hg.mozilla.org/comm-central/rev/b7b3bdecb432
Replace initKeyboardEvent with new KeyboardEvent. r=freaktechnik