Dude, you need Firebug. www.getfirebug.com

It's replaced Venkman for me. And it's got SO many nice features. Perfect companion to Web Developer.

I also use:

IE Tab => for easily switching into IE from within firefox (just click the little logo in the status bar)

Adblock Plus => obvious
Adblock Filterset.G Updater => updates filters automatically

FLST => focus last selected tab. close a tab, and it goes back to your previously select one. click on your current tab to toggle quickly back and forth between two tabs.

Google Toolbar => love the autocomplete, and other quick functions available

Google Bookmarks Button => i like this because i use google bookmarks (instead of del.icio.us or something) http://www.menjatallarins.com/extensions/