The 'charCode' property of a keyup event should not be used etc.

This message: "The 'charCode' property of a keyup event should not be used. The value is meaningless" appears every time I type into a form while debugging it.

Is this a spurious message in Firebug, or is there something I can fix in my code to prevent it? It takes up lots of real estate and causes inefficiencies while debugging forms. If it's something I can fix - just say so and I'll pursue independently. However, if the error message doesn't mean anything, can it be removed?

Thanks.