Interesting theory, but I don't see it on my computer.
In the regular reply box, I have remove text formatting, bold, italics, underline, color, insert link, insert image, wrap (quote) tags around selected text, increase/decrease size, and switch editor mode.
In the advanced reply box, I have the same things plus font, size, smilies, attachments, undo/redo, alignment, numbered lists, billeted lists, remove link, insert e-mail link, wrap code around selected text, wrap HTML tags around selected text, and wrap PHP tags around selected text.
There is simply no icon or menu item for spell checking, not in the upper right or anyplace else on the screen.
I'm using Firefox 3 running under Vista 64BIT SP2, if that makes any difference.
What am I not seeing?