How about exposing an API for dev tools similar to the "swank" protocol that Emacs uses for editing Lisp code? I know there are people doing stuff like this already, but it would obviously be a lot easier if there were a nice protocol for it (ideally standardized across browsers...). I want to be able to seamlessly interact with a running Firefox instance from my editor or IDE, sending it snippets of code to evaluate and seeing my page change in real-time.