If I recall correctly, Firebug had to do nastier and nastier black magic in order to hook into the correct parts of the rendering engine and provide the data it

If I recall correctly, Firebug had to do nastier and nastier black magic in order to hook into the correct parts of the rendering engine and provide the data it had. This meant both that Firebug was a poor experience to develop for, and also that it constrained Mozilla to not break certain internal interfaces, even though they weren't designed for public consumption. A tool like this really does need to go into the browser as a first-class thing, because the information it needs access to is very low-level. So, the Firefox team is making their own. As a side benefit, it should be much easier to write, since they have the power to actually change Firefox to make the information they want easily available.