Button Click Event Not Working With Firefox

Following is the code that i am using. It work in IE but the button click event is not generated properly in firefox:

function trapEnter(btn,hdn, event) {
var key;
var isIE = true;
debugger;
if (window.event) {
key = window.event.keyCode; //IE
isIE = true;
}
else {
key = event.which; //firefox
isIE = false;
}
if (key == 13) {
var btn = document.getElementById(btn);
if (btn != null) { //If we find the button click it
document.getElementById(hdn).value = '1'
btn.click();
key = 0;
}
}
}