How to set a text value for document.activeElement?

Question

Knowing that my document.activeElement is an input field (I don't know exactly the name of the component, but may be the Google's search input field, for example), how can I set a text on it programmatically?

--update

I'm trying it from a xul application, via javascript after the page is loaded. A paste command works fine, so I know the field have the focus. (and I didn't put the Xul tag becouse it's just about the javascript)

Answer 1

See the mozilla reference. This is the same type as document.getElementById()

document.activeElement.value = 'new value';