I discovered a fantastic trick for getting the spell checker to work in virtually all text fields, not just input boxes.
As usual, open about.config in a new tab/window, and locate layout.spellcheckDefault. Change it's value to 2 to turn on Firefox 2's spell-checking in input fields as well as text areas.
You'll need to close and restart Firefox for it to take effect. Oh, and I find it amusing that the Firefox spell checker flags Firefox as a typo.