We currently using FCKeditor 2.6.6. However this version doesn't work well with internet explorer 9. Most of buttons which creates pop-up windows doesn't work in ie9 (paste, flash, image, etc.) and no pop-up window is showed or only empty pop-up is showed and fckeditor freeze.

According CKeditor official pages this version of FCKeditor isn't supported and to fix problem is suggested update to new version CKeditor 3.x (atleast 3.5.2 which added ie9 support). So we should update to later version where this problem are fixed.

As workaround in ie9 is possible add to head of pages:
<meta http-equiv="X-UA-Compatible" content="IE=EmulateIE8" />
this cause changing of rendering engine in ie9 to ie8 where FCKeditor work well.