Hello.
I need to check keypresses in document, so I used
document.onkeydown = checkKey;

with this function, that looks like:
function checkKey() {
if(event.keyCode==16).....
}

it works fine in IE & Opera but not in Firefox's Mozilla.
What should I do to get it work in all browsers, even in mozilla?

use event.charCode for firefox.

https://developer.mozilla.org/en/DOM/event.charCode

function checkKey() {
var CKey = event.charCode;
}
CKey - undefined in all browsers

var CKey = e.charCode - same result