After recent auto upgrades, I have found the inline Firefox spell check function not working at all. There doesn't seem to be anything on the Net on this either. Eventually I figured out that somehow the dictionaries were not installed.
By right-clicking in a text field where you'd expect the spell checking to work, you'll see "Add dictionaries" option. Upon addition of the dictionaries, the FF requires a restart and after that everything works again :)
Good luck...
