If I may, here is a Firefox compatibility fix:

Issue - Firefox back button is invoked when pressing delete and the combobox has a zero-length string.

Cause - Firefox does not recognize the ‘window.event’ object, instead it looks for the event to be passed through the function parameter.

To resolve - Modify the onkeydown event wiring as follows:

document.onkeydown = function(e) {
var windowEvent = (e) ? e : window.event;
if (combo && combo.editing && windowEvent && windowEvent.keyCode == 8) {
windowEvent.cancelBubble = true;
windowEvent.returnValue = false;
if (combo.insertSpace) {
combo.insertSpace = false;
}
else {
with (combo.options[combo.options.length - 1]) {
text = text.substring(0, text.length - 1);
}
}
}
}