Just created a bugzilla issue: https://bugzilla.mozilla.org/show_bug.cgi?id=1634653

In Firefox the result.insertAdjacentHTML immediately returns and doesn't wait until the iframe has been created. It works if I place the code in the timeout function.

function haha() {
  console.log('haha')
  const result = document.querySelector('#result')
  
  result.insertAdjacentHTML('beforebegin',  '<iframe id="iframe"></iframe>')
  
 setTimeout(() => {
 const el = document.querySelector('#iframe')
  const doc = el.contentDocument
  
  doc.body.innerHTML = 'Something'

result.innerHTML = [
      el === document.querySelector('#iframe'),
      doc === document.querySelector('#iframe').contentDocument, 
    ].join(',')
  }, 0)
}

const btn = document.querySelector('button')
btn.addEventListener('click', haha)

@cor-el Thanks. Please have a look at this issue: https://bugzilla.mozilla.org/show_bug.cgi?id=543435

If your PC is a little bit slow, you will see the iframe show content first, then changed to blank.