If you have upgraded to FF 23 and suddenly your CKEditor is not working (in admin->catalog->categories/products), take heart: it’s a one line fix to get it back again. Edit
<your renamed admin>/includes/ckeditor.php
and change the inclusion of jsapi to not use a protocol, i.e.
and you should be all set. You will NOT see this issue in FF 22 and prior, only FF23 (and perhaps higher).
NOTE: The latest CKEditor package in the ZC Plugins area already has this change in it.
This was originally posted by That Software Guy on the Zen Cart forum on 8/28/13.