One of my favorite developer extensions has finally been “updated to work in Firefox 2.0”:http://karmatics.com/aardvark/.
Although it’s not as full-featured as “Firebug”:http://getfirebug.com/, I find that I prefer to use Aardvark as an element inspector — it’s a lot simpler to use and is easier to jump in and out of.
Make sure you learn a few of the keyboard shortcuts that go with it to really get the most out of it.
Want more extensions? View my previous post on “useful Firefox extensions”:http://www.smileycat.com/miaow/archives/000281.php.