I ran into this problem a little while ago as well, I found the following post on the FCKEditor forums:
viewtopic.php?f=6&t=13886
That's usually some extension that it's misbehaving, so the debugging procedure is to start Firefox in safe mode and verify that it works correctly. Then start enabling the extensions one by one until you find out the culprit. 
in my case, it was The Browser Highlighter add on that was causing the problem:
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