Firebug, Joe Hewitt's long-awaited Firefox extension for debugging DOM/CSS/JS/Ajax projects has finally hit public beta, and is available for download as of today. If you're doing any work tweaking the DOM via CSS/JS, or general Javascript development, this extension is - quite frankly - a must have extension for your workflow.
Firebug takes some of the favorite staples of the Web Developers' Toolbar (link below) such as CSS/HTML visualization, the ability to introspect your HTML/CSS/JS assets, and adds some must-have debugging tools like an integrated JS debugger/logging tool, network bandwidth profiler (see how all the assets in your page contribute to page load), DOM inspector, and a great error-handling UI to manage all the tricky bugs in your code (you do write the occasional bug, right?).
Alongside the old-favorite Web Developer's Toolbar extension by Chris Pederick, Firebug should become a staple of your web dev toolchest in short order, and in my opinion- the first thing you install into Firefox. Hope you find it equally as useful!
omg it rocks