I was getting a situation where, both in FogBugz and in the CKEditor demo page, the editor would not show up. Spent hours going through all sorts of things. FF 5, Add-ons turned on, Add-ons disabled, nothing changed the situation, until on the Demo page, I went to their tab to change the language.
Their demo list was set to the first thing in the list, Africaans. I changed it to English, and the editor appeared as it should. Though if I switched to any of their other tabs, it would fail and just be a big div full of whitespace again.
Anyway, I went into Firefox's Tools->Options->Content tab and noticed that I had no default language set. I set this to English and now it seems to be working OK.
I reported my problem in the CKEditor forum, and I will go there now to provide this update to them as well.