IE vs FF window.event issue

Trying to capitalize the letters as the user types in the text box, works fine in IE but not in Firefox

Firefox, and other real browsers, use the standard Event instance as defined by the W3C. IE, on the other hand, uses its own proprietary layout. You need to accommodate both (or use a library like jQuery which normalizes the event object).

Most browsers will not let you reassign the key code for security reasons. You are better off doing it after the user type it in.