But it's all frankly out of date, if you're into development you need to use Firebug, since I've owned a mac that's pretty much all I use, I dislike the Web Developer toolbar but I'm sure there are advocates of it here.
A great tool for power users is Smart Search It uses your keyword searches to allow you to search any selected text using your context menu in something other than Google, I use this all the time, it's well handy.
Flashblock, Adblock plus, PDF download, The nuke images/anything/plugins extensions. I usually have FireBug, an extract-video-from-flash extension and a mouse gestures extension as well, though not right now. (They were not ff3b5 compatible yet last time I checked.)
They offered me a job for something I did that only worked in FireFox ;)
To be honest, it took me quite some time to find my way with Opera. It's clear that working for them, meeting people who know it in and out, helped a lot to find out many practical/useful things I wouldn't have had the patience to look for otherwise. Opera has a heck of a lot to do in UI and UX to be more new users friendly.
As a power user I find it annoying that it takes quite a few extensions, that are incompatible with updates of FF, to get the best of it. But there is absolutely no doubt for me that FireFox is a solid, fast and accessible browser.
Aardvark
Colorzilla
DOM Inspector
DownloadHelper
DownThemAll!
Firebug
Flashblock
Image Zoom
Menu Editor
Mozilla XForms
Nuke Anything
OpenBook
text/plain
Titlebar Tweaks
User Agent Switcher
Web Developer