Getting And Setting Key Character In Key Event

I have had this function work perfectly in IE and am trying to get it
to work in Firefox. I have seen plenty of questions and answers on the
web for how to get and check the key pressed but nothing about setting
or changing it. I only want uppercase characters in this case. If the
key is a lowercase, it needs to be changed to an uppercase character.
IE lets you use keyCode to change the character code. How do I do the
equivalent in Firefox?

<textarea cols="40" rows="3" name="Rule" id="Rule"
onkeypress="checkUppercase(event);"></textarea>

function checkUppercase(e) { .....