I had searched for info and found that thread on CS forums before posting here. Following your suggestion, I've posted a reply/question to that site, and will let you know if/when I get an answer (asked about options for the user and admin). I noticed that I did not get the use of FF spell checking feature while composing the post on that site either (and I expect they have latest updates - so unlikely a version issue).
If this fails, consider adding this question to a future support ticket to CS. Like the direct image paste issue, this is not critical, but would be a useful feature.
Just for reference, I'm using FF version 3, that was released this week (and had been using a beta of V3 on another system). The FF spell checker uses a red underline to indicate errors; and Rt clicking offers spelling suggestions and options to ignore or add to dictonary. There is an advanced option to allow the feature to check small text boxes (and doing that had no effect on CS forum post composition).
Any notes on the "Enhanced editor" option in profiles section? I didn't notice a difference over "Standard."