If impossible to modify header in JSONP. How does twitter extension in Chrome work?

Question

Now I'm working to do twitter client with Javascript.
After I read this topic, I have question

Modify HTTP Headers for a JSONP request

He told impossible way to modify HTTP Header when using JSONP.
If it's true. How does twitter extension work in Chrome ?
I think Chrome extension use Javascript, too.

Answer 1

Extensions have more privileges then "normal" JavaScript code executing within a page. Check here:

• http://code.google.com/chrome/extensions/xhr.html