I welcome the changes to the CSS tools in particular! While the Inspector is full featured and offers some capabilities not found in Firebug, Firebug still feels more natural and reliable to me. The change to show unfiltered CSS rules is really helpful.
It's odd that Mozilla hasn't officially adopted Firebug as part of Firefox. While Firebug continues to improve, and the Firebug team does a great job, with Google is focusing on this area I suppose at some point I will prefer the Chrome inspector. Why is Firebug still a plugin?
On the darker side of web development, IE's development tools are typically lacking. Does anyone know whether they have been improved in IE 9?