This is a repost of Spell-checking in question titles on Meta.Gaming, except that this post furnishes the necessary technical details for it to work.
input type="text" elements, by default, do not have have spell check enabled on Firefox. This option can be turned on by the client by using the right click context menu and checking the 'Check Spelling' option.
With HTML5, there is now the
spellcheck attribute. As of March 2009, only Firefox supports this. However, there is no harm in turning it on, and so it should be turned on, if only for the Firefox users out there, and as more and more browsers adopt this attribute, this should be fairly useful for users.