Firefox Extension - Add Element to Body

I want to add a element to 开发者_如何学编程the body in my Firefox Extension per javascript. document.write("ads"); doesn't work. - no result in the sourcecode. I'm sure the JS is executed, because I have an alert(); on this place. It's possible?

/edit: have to use content.document.write

You should use this code to inject element into HTML:

document.body.appendChild