Quoted from the FCK website:
"Safari has some limits that goes beyond the editing features and that are key things to make the editor work. For now, I’m stumbling across one of the most important limits: it is not possible to load scripts dynamically (used for custom configurations, plugins and the editor code itself). I've reported this problem to the WebKit developers. You can read it here (http://bugzilla.opendarwin.org/show_bug.cgi?id=4403).
I've also started some contacts with Safari's developers and introduced my intentions in their mailing list. You can find it here. They have their availability limits and priorities, so things are going slowly but I think we will have success with it. I would also like to thank Maciej for his interest on FCKeditor.
You can help use by letting Safari and WebKit developers know that this effort is important to you. It will be good for both parts, for FCKeditor to the compatibility and for Safari to be an even better browser.
And just to remember, on Mac, FCKeditor is fully compatible with Firefox, so you can use it for now."