When I wrote Firefox 4 begins to take shape, Heads Up Display (HUD) — for web developers wasn’t there, it was pushed in days after the post.
Heads Up Display is supposed to, (as presented on the projects page)
Above all this ‘interactive console’ will replace the existing ‘Error Console’ window that is still present in Firefox 4.0, provide an
interactive JS editing environment, a rich logging experience for web developers, and a simple object inspection (either via console or separate UI).
As Firefox 4 approaches its first beta, more information on this major update will be pouring out, come back for more. Also expect new features, and lots of polishing. HUD is present in both Linux and Windows builds of Firefox 3.6 Alpha 6. Keep an eye on the project at Firefox/Projects/Console