FireBug - A great new Firefox Extension for Web Developers...
I find this nifty Firefox extension over on the Digital Media Minute blog. The FireBug extension is basically a combination of a DOM Inspector, JavaScript Console and XmlHttpRequest packet sniffer—all wrapped up into a Sidebar component.
I haven't played around with it very much yet, but for a "pre-pre-alpha" seems to work very well. I did notice that the keyboard shortcut to show the sidebar, does not hide it if it's already visible. That's the only thing I've found so far.

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Plus, the FireBug was showing errors produced by my other FireFox extensions... which I thought was strange.