its still Beta quality (its only in the nightly builds), and as far as I don’t know it doesn’t have a plugin architecture like Firebug. But the bones of it is there:
Element inspector, CSS display, CSS Editing editing selectors, adding new , live html editing, deleting elements etc.
Read about the improvements here:
Get WebKit here:
C’mon Microsoft, step up your game, IE is way behind the pack here.
if it wasn’t for their market share, I would refuse to support IE.