[logn]

Every browser already ships "developer tools" for inspecting and debugging webpages. And you can already use those tools to rewrite the pages. I see this as the next step of that. And audio or word processing tools are less related to the web than this IDE. I agree that I don't see the harm in making this an Add-on instead. Anyhow browsers are the new operating systems, and Firefox has a development IDE bundled.

[alxndr]

Chrome has an option where you can save your edited JS back to the file system; it also feels like another step along this path.

I also think that including dev tools and even an IDE like this in a browser are fundamentally different from including a mail or calendar client in a browser.