Key events generated by triggerKeyEvent() function (htmlutils.js file) have always both keyCode and charCode properties assigned with the same value (the code of the desired key). But the actual behavior of Firefox in these circumtances is to assign only charCode for visible keys and only keyCode for special keys (esc, tab, f1, f2, ...).

File htmlutils.js, line 310/311:
   evt = document.createEvent('KeyEvents');
   evt.initKeyEvent(eventType, true, true, window, controlKeyDown, altKeyDown, shiftKeyDown, metaKeyDown, keycode, keycode);

The last two parameters passed to the function above should be mutually exclusive (at least for Firefox).