I’m currently using TabWarning (because I’m an idiot), Tabbrowser Extensions (which is really useful for getting Firefox to act like other tabbed browsers I’m used to), TabX, Copy Plain Text (mentioned in the Wired article), and DictionarySearch (ditto).

Lol!  I just installed Mozilla yesterday and immediately discovered the extensions while searching for a way to implement my favorite browsing features from Slimbrowser.  I stumbled upon this site:
MozillaTips.com
Which had a link to a tag extension which rocks.  I am now in love with Mozilla.

I also found this blog entry pretty useful: a really good post on extensions that pumps Firefox/Mozilla up from barebones browser to sand-kicking-in-face strength.  Very cool.

Those are all good.  I personally like CloneWindow, EditCSS, Googlebar, Focus Last Selected Tab, Last Tab, and naturally, the Bloglines Toolkit.

I’m currently using:
Web Developer Toolbar (lots of handy tools for diagnosing problems with web pages)
Tabbrowser Extensions, because I like being able to re-order tabs and to re-open a closed tab
Firesomething, for a bit of variety
Gmail notifier, so that I can keep tabs on my Gmail account
BBCode, makes entering BBCodes in forums that support it a bit easier
GMailCompose, lets you select an email address and open it in Gmail.

Tabbrowser Extensions.  Unless firefox has done away with its Google search bar with 0.9 (I’m still on 0.7) then that’s all I need.

captcha = light.  How appropriate.

OpenBook
All-in-One Gestures
Single Window
Basics
Bookmark Backup
Session Saver
stumbleupon

I just began using FireFox last week and it’s been great.

StumbleUpon is a blast for a little downtime.
SpoofStic works great for redirected URL’s
Tabbed Brower Extensions
Double-click on
BugMeNot is wonderful if you visit a lot of news sites.
Nuke Image/Nuke Anything makes almost any site SFW.
Dictionary

I’m curious as to what Diggler does and haven’t played wth it yet.

Overall, I use:
AdBlock - Ad blocker. Works with wildcards!
Flash Click to View (called FlashBlock now I think) - Makes it so Flash doesn’t just play. You have to click first.
Nuke Anything - Right click and remove anything. Useful for nuking images that are too large and screw up layout
JS Console Status - Adds a little red icon when there’s a javascript error on a page

I wan’t to thank everyone for their posts here. Since I had been relying on the list of extensions that you get through Tools > Extensions in Firefox 0.9, I probabably would not have found Tabbrowser Extensions on my own. Although the interface is a little different than that in NetCaptor, this extension gives me pretty much the functionality that I have been looking for.