Browser development?

I'm wondering, how can I develop application/plugin/browser extension that can extend functionality of search e开发者_如何学Gongine, like McAfee SiteAdviser puts green checks or red crosses on Google page when it thinks the web-site is 'clean'

https://developer.mozilla.org/en/Gecko_Plugin_API_Reference

You get access to the DOM and can do stuff to it.

The Internet Explorer Development site is at http://msdn.microsoft.com/en-US/ie/.