I would say the problem here is that Firefox needs to use the system spell checker. They can't just "expand" it. It needs to support per-user custom dictionar

I would say the problem here is that Firefox needs to use the system spell checker. They can't just "expand" it. It needs to support per-user custom dictionaries. That's the same complaint I have, BTW, with every part of Firefox. If it's part of the UI, I don't want them to try to do better. I want them to just use the built-in one.