How about opening up a browser-based API for existing in-browser IDE-like environments to leverage the debugging tools that are already present?
What I am talking about is instead of recreating jsFiddle in the dev tools, how about letting jsFiddle (with appropriate user confirmation) control the dev tools?
As the creator of Plunker (http://beta.plnkr.co/edit/e5iLyQ?p=preview), I would love to be able to leverage the environment that has already been created to allow adding breakpoints and inspecting of running code. Since Plunker already has live-previewing and a working multi-file editing interface, it would be really amazing to be able to take it to the next level.
Shameless plug: If you've never heard of Plunker, check it out. For free you get live-reload of previews, transparent compilation (and sourcemapping) of stuff like Coffee-Script, Typescript, Stylus, etc.., real-time collaboration on the same code, live js/css linting and much more! Also, it is open-source (http://github.com/filearts/).
What I understood from your article is that you're mostly exploring 1) the possibility of opening up the dev tools to locally-installed applications like SublimeText2; and 2) the integration of jsFiddle-like apps within DevTools.
Please correct me if I misunderstood.
What I'm asking about is creating an in-browser API for a website to interact with the dev tools. Obviously, this would require a very thorough review of the risks of having such an API from a security perspective (and how users can opt-in in a transparent and well-controlled manner). That being said, I am still hoping!
If this is already in the books, then Hooray! I can't wait to start experimenting.
I hope I didn't come across poorly, because I really appreciate all the work you've been doing!
So - Firefox exposes a tcp-based protocol (not enabled by default. The user will need to go to a special tool to start the server). It will expose feature like adding break points, exploring the DOM and the CSS rules, and editing content (CSS/HTML/JS).
So external tools can connect to Firefox. But this protocol is not accessible from a page. But we could imagine using a websocket instead of a normal tcp socket, and let the web page connect to ws://localhost.