Firefox 50.1.0 focus to iframe (visible or zero w/h) if its src destination has SSL certificate problems (eg. expired), needs slowdown rendering to trigger onload after iframe loaded (eg. with advertisement, synchronized script etc.)

Problem seems to be from code of aboutNetError.xhtml template page, which contains autofocus="true" arguments on two <button> elements on source lines 594 and 596 in this code https://dxr.mozilla.org/mozilla-beta/rev/9ddd4fee07842e72ba49f1583ec5f596f6e60e72/browser/base/content/aboutNetError.xhtml