Although Firebug has "fire" in the name, it is not an official Mozilla project. It's an independent group of developers (though naturally, many are Mozilla contributors). That independence is important, as competition always is! I can't speak for the developers who are working on it, Firebug will likely evolve to be a complementary set of tools to the built-in ones. Today provides valuable continuity for people who are used to using Firebug to build websites, and has a number of addons for Firebug itself that exists nowhere else on the web. Shutting it down would be a shame.
