...is now a stable release. It's notable because it's the first release with it's own built in developer tools (video overview). It's all HTML/CSS for now (not a Firebug replacement) but they are nicely designed and a great first step for a product that really should be in-house. Also, CSS 3D transforms! After you've upgraded, check out this demo using -moz-element and the just-now-supported 3D transforms.