Yes, yes it is. And after using it extensively to help redesign my current template so it’s all CSS and works in all the browsers I found out how awesome it was.
No more need to use dreamweaver for anything, firebug can do it all within the browser and now I think it’ll become another one of my 100% install everywhere I go extensions much like colorzilla, measureit, and web developer toolbar
Indeed! I’m most enamored of the CSS viewer/editor, but, then again, I write webpages in vi; WYSIWYG mostly happens to other people. 
Great link. That’s definitely a must-have extension.
It had me at “click an element and see all the CSS rules that are affecting it,” and that’s not even scratching the surface.
And I thought the Web Developer extension was cool.
I agree wholeheartedly.
@Mark Jaquith: I know, its pure genius…
I find more and more useful everyday. Note the Beta 5 release was the first to include “support for Firefox extension updates” so be sure to upgrade if you’re using 4 or below.
ps - @scud: you may be interested in Firefox Portable.
I recently started using Firebug and love the xmlhttp header debugging feature. I’m lovin it. 
If you like Firebug you might also give HTML Validator for Firefox a try. It’s based on Tidy by the W3C and works quite fine for HTML Validation.
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