If I add the IE Spell button by itself on a toolbar then all the buttons are hidden when viewed in Firefox or Chrome.
I get the not very helpful error:
Timestamp: 07/05/2013 10:30:29
Error: TypeError: k is undefined
Source File: https://<mysite>/<myblog>/wp-includes/js/tinymce/wp-tinymce.php?c=1&ver=349-21274
If there are any other buttons on the same toolbar as the IE Spell button then it works without any errors.