Detecting Keystrokes In Firefox?

I'm trying to detect keystrokes using Javascript. My code (so far) works in Opera and Safari, but not in Firefox, Netscape/SeaMonkey/Mozilla, or Chrome. (I have no idea if it works in IE; IE has other problems with my code.)What I've done is attached a onkeydown listener to the body tag. (I've also tried it with onkeypress and onkeyup.) Right now all I'm trying to do is detect a key press -- any key press -- and throw an alert as a result. I've tried it two separate ways -- with straight Javascript and with MooTools, which I'm using for the rest of my project.

The code I have is as follows:

mootools version
$(document.getElementsByTagName('body')[0]).addEvent('keypress', function(event) { alert('key has been pressed.'); });
regular javascript version
document.getElementsByTagName('body')[0].addEventListener("keypress", function() { alert('key has been pressed.'); }, false);

Both versions work in Opera and Safari. Interestingly enough, if I attach a click listener to the body using the exact same code as above, but changing the word 'keypress' to 'click', they both work in Firefox as well. I can't figure out how to get this key listener attached.