Firefox Gives AJAX error when it returns empty

I'm pretty sure this isn't 'just me'...but Firefox gives me an AJAX error when the submit button is clicked:

I tracked this back to the AJAX submit function: simple_cookie_compliance_dismiss_form_submit() where it returns:

  // The ajax call wants something to replace HTML-element with. Since we in
  // this case want to remove it, we replace it with nothing.
return '';

I fixed it by simply returning a hidden span instead like: return '<span class="element-hidden"></span>'; and my error went away (and everything was hidden in the result as expected.

Can anyone else verify this problem?