I have noticed that unless the server side supports these online spell checkers as you mentioned, the option isn't available with the media, such as a message boards and so forth.
I have run Spellbound with Firefox and Mozilla with no degradation in performance but I don't know exactly how it works. Of course, it is an optional plug-in no different than shockwave and the likes. You will be surprised at the number of message boards and forums that don't have spell check options available. Even IE has the popular independent plug-in speller called, ieSpell. Just something to think about for the future. Spellbound would be the best route since it is already created for FireFox and Mozilla.
Actually, it looks like it is an extension. Perhaps it can still be installed into KM?...
http://spellbound.sourceforge.net/download