insertAdjacentHTML() doesn't work in FF

[Sammeeey]

This part of the code in Popups and window methods doesn't work in Firefox:

newWindow.onload = function() {
  let html = `<div style="font-size:30px">Welcome!</div>`;
  newWindow.document.body.insertAdjacentHTML('afterbegin', html);
};

It doesn't insert html to the document.
In Chrome it works for me.

If other can confirm, then what would be a good way to edit the tutorial?
Maybe just add another caution box with a respective hint (saying that it doesn't work in FF?)