I really, really don't get this.
The web browser is a client application for viewing web pages. The vast majority of users of web browsers don't want or need development tools built in, and it just adds to the bloat and slows the browser down.
Firebug is a great diagnostic add-on tool for tweaking CSS or finding obscure errors, but why try and use it as a development tool?
If you're a developer, then either use an IDE, of which there are hundreds (why only mention two of the worst, Dreamweaver and FrontPage?) or just use a text editor (with syntax highlighting if you want), a browser and a local instance of a server.
Is that really too complicated for the modern web developer?