IE 10 is the first browser where the major version number (before the dot) is in double digits. Most scripts, including the one written for FCKEditor at the time had a browser detection regex pattern that expected only a single digit before the dot (see below)
var sBrowserVersion = navigator.appVersion.match(/MSIE (.\..)/) ;
The following change will thankfully remediate the situation immediately.
var sBrowserVersion = navigator.appVersion.match(/MSIE ([\d.]+)/) ;