The default argument to HTMLTableRowElement.insertCell() should be -1, not 0, according to the specification:
Firefox 29 and IE11 both match the specification. Blink is about to change its behavior as well:
Created attachment 230920 [details]
Comment on attachment 230920 [details]
Clearing flags on attachment: 230920
Committed r168390: <http://trac.webkit.org/changeset/168390>
All reviewed patches have been landed. Closing bug.