Those two methods are deprecated in Firefox 7 because they have been removed in DOM4, the current version of the DOM spec. It is likely that eventually they will be removed from Firefox and possibly other browsers.
If the warnings are coming from CKEditor code, I'm sure the CKEditor team will be aware of this and release an update at some point that does not use these methods. However, I couldn't find any use of this method in CKEditor's source code, so it's more likely to be coming from something else you're including on your page. Possibly CKFinder, which does seem to use that method?
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