Rated 5 out of 5 stars
Hugely useful, absolutely indespensible extension.
Re. the review below: hardly a fair rating, even if the complaint was wholly accurate, which it is not. While Firebug 1.2 has lost the convenient global disable/enable-per-site option it had before, in its place is the option to separately disable the console logger, script debugger, and network monitoring sections, each of which can be disabled globally and then re-enabled per-site. Very useful indeed if you only ever use the HTML/CSS trace features and don't need the script debugger slowing everything down. If you're still concerned about the performance impact of the HTML/CSS/DOM inspectors (I have no idea what this is in reality) you can always disable the extension when not developing, or just run a separate developer profile as I do.