Firefox 2 Spell checker

I am having trouble getting FF2 to stop flagging mispells of words like “honour”, “favourite”
which are correct English
(it objected to the above use, for example)

Although I can “read” American, it is a nuisance, since I think and type in English, and I have my keyboard trained for that 😉

I have the British English add on, so what am I doing wrong?

Do I need Harry Potter to use a spell to invoke the Dictionary?