UserAgent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:55.0) Gecko/20100101 Firefox/55.0

Steps to reproduce the problem:
1. Navigate to this address:

data:text/html,
<form action="https://www.mozilla.org/" id="form">
  <input type="submit" value="Navigate maybe">
</form>
<script>
  var form = document.getElementById("form");
  form.addEventListener("submit", function (event) {
    console.log(event.cancelable);
    event.preventDefault();
  });
  form.dispatchEvent(new Event('submit'));
</script>

What is the expected behavior?
The form is expected to navigate away to https://www.mozilla.org/ (form.action)

What went wrong?
The form doesn't navigate away.

I'm not sure if this spec-violation is a bug, or if it's intended as a convenience to developers (I agree that my code looks like I'm trying to make the <form> not navigate away).

Did this work before? N/A 

Does this work in other browsers? N/A

Chrome version: 58.0.3029.110 (Official Build) (64-bit)  Channel: stable
OS Version: 10.0
Flash Version: 

In my "submit" handler, `event.cancelable === false`, and `dispatchEvent()` returns `true` (i.e. "event wasn't canceled"), however the form still doesn't navigate away.

Firefox honors the spec behavior, so I've filed a Firefox webcompat bug: https://bugzilla.mozilla.org/show_bug.cgi?id=1370630