This is a problem that occurs when the /extras folder is storedon another domain.  We tried to put our /extras on our CDN (akamai).  Available at a specific hostname.  We put the hostname in our site's config file.  We had no problems whatsoever in Firefox, but when it came to IE, it borke with some JS errors related to Tiny MCE. 

What the user saw when they went to edit a page/article in WebGUI was the form would load, tinyMCE would appear to load, but nothing could be typed in any of the fields.  There would be IE's JS error message in the bottom left corner of the browser window.

In terms of severity, we think it should work consistently in all popular browsers- IE/Firefox/Safari/Opera.  If TinyMCE is the issue, then perhaps that have some workarounds or alternative configs we can try.