even.keyCode=9 tab event donsent work in FireFox

I am having trouble in calling tab event using my own function.
i call tab event using
event.keyCode = 9;
but firefox gives this error "Error: event is not defined".

What to do? plz help.

You have to pass the event object to the function. That's the cross-browser way to do it.
function fKeydown(evt){
alert(evt.keyCode);
}
<input type="text" onkeydown="fKeydown(event)" />