Get and modify browsers incoming and outgoing traffic

Heres a tricky one...
I need to write an application which contains a browser. However, I need to get the outgoing traffic from the browser and modify it before sending it off over the internet to the server. Likewise, i need to grab incoming traffic and modify it as necessary before sending it to the browser.
I am wanting to write a couple of programs this way; one which is a form filler that can fill a form and submit it without the form even appearing in the browser, and the other as a security app to stop suspicious/annoying cookies and prevent malicious scripts running.