[Cthulhu_]

Well that's how developer tools for Firefox started out as (Firebug), however that was also dependent on a plugin API that gave access to all of those things; by building it in natively, the dev tools have access to all of the APIs, not just the ones exposed by the plugin APIs.

[codefisher]

That is not really very relevant when building a Firefox add-on. Firefox add-ons get incredible access privileges to the browser internals. That is part of what makes it hard, but also powerful. An example of what I mean, it was not overly complex for Sunbird (the calender app) to be turned into a add-on. There is no reason also for example that Thunderbird could not be turned into an add-on for Firefox. Except then you would have SeaMonkey...

But my 2c, is that even if the project does not work out as hoped, what ever they are doing can put turned into ad add-on.