Yes, the same thing has been happening to me on a sporatic bassis. It started about a month or so ago and it seems to be happening less often. I haven't a clue as to why it does it but I hit the browser back button and then "spell check" again and nine times out of ten it works as it should. It started about the time I upgraded my FireFox browser to 3.5.5, of course that may very well be coincidence.
Regards,
GtG