OK, organized as best as possible by my current usage:

Active and I couldn’t live without them:
SessionSaver .2 0.2.1.027 – “SessionSaver will auto-track and restore your browser exactly as you left it — every startup, every time. Not even a crash can faze it…” – this is my #1 extension and absolutely essential. Never browse without it. On a fresh install, this is my first download. Get it now.
Adblock 0.5.2.039 – does what it says. Pretty cool.
Download Manager Tweak 0.6.3 – makes it easier to control your downloads.
Add Bookmark Here 0.5.2 – Makes filing bookmarks easier.
FLST 0.7 – Brings focus to the last selected tab when closing the active tab.

Active and used every day:
ScrapBook 0.13.7
Duplicate Tab 0.5.1
PrefBar 2.3.1
lori (Life-of-request info) 0.1.0.20050201
CTC 0.3 – Close Tab Context (menu) puts an option in the context menu to close the active tab. Very handy.
LastTab 1.0.5 – modifies Ctrl-Tab to switch to recently selected tabs, rather than always go from left-to-right.

Active and used occasionally (but still important):
Copy Plain Text 0.2.1
undoclosetab 20041125.3
Web Developer 0.8
Live HTTP Headers 0.10
SubmitToTab 0.2.1

Nice-to-have:
Copy Plain Text 0.2.1
ReloadEvery 0.5
DictionarySearch 0.7
Configuration Mania 1.05.2004110501
Rot13 Encoder/Decoder 0.1
Calculator 0.7
ieview 0.84 – launches current page (or link) in IE
Launchy 3.9.0 – Launches the page (or source) in various places, including IE. Like ieview on steroids.
FirefoxView 0.31.1 – the reverse of IEView… adds a context item in IE to view the page in Firefox
Checky 2.5 – Checks the HTML/CSS against various validators
Ext2File 0.2.8 – Exports an extension list to a file (so I could make this post!)
Open Source in Tab 0.1

Played with and then haven’t used:
Plain Text Links 0.2
Linky 2.3.0

Tried to use but haven’t figured out yet:
Magpie 3.1.2
BugMeNot 0.6.1

Not used yet but sounded cool:
Keyconfig – assignable hotkeys for your favorite stuff
DOM Inspector 1.0
LoremIpsum Content Generator 0.4.1
Sage 1.3.1
Nuke Anything 0.2
jsLib 0.1.245
Minesweeper 0.8
View Cookies 1.2
Mozilla Calendar 0.8.3+
Calendar Help 0.1
Feedview 0.9
RequestPoison 0.5
EditCSS 0.2.3
Drag And Drop Upload 1.0
Download Sort 2.5.0
Leech 1.5.0+
webcache 0.1
Googlebar
Privacy Fox 0.3.7 – View short privacy policies for sites
Policy Manager 1.3.2004081801 – allows you to set Javascript (maybe others?) policies for specific sites, like the Zones stuff in IE.
Right Encoding 0.1 – Lets you see a page’s encoding (language and character set) and manipulate the selection. Should be useful if you’re debugging a page that appears odd in other character sets?
SwiftTabs 0.2.1 – move to next or previous tab via keystroke. I just haven’t learned the keystrokes yet…

Didn’t really work out (but not their fault):
spooFX 0.3
SpoofStick 1.05

Just plain didn’t work like I wanted:
Research Buddy 1.01
Slogger 0.5.11

Hated it!
RadialContext (and I can’t get it uninstalled!)

I don’t know…
PrefButtons 0.3.2 – I may be using these buttons but I’m not sure… too many extensions!

# DISABLED EXTENSIONS: (5)
Bandwidth Tester 0.4 – out of version sync
miniT (drag+indicator) 0.4 – out of version sync
Toolbar Enhancements 0.16.2 – out of version sync
Tab X 0.5 – Messed with my tabs closing, and I couldn’t have that
ForecastFox 0.5.9 – Would make my browser lock at 100%, usually when I was offline. Might have been something about being disconnected, but on a portable machine that was catastrophic because SessionSaver is still only 85-92%.
Retro Find 1.1 – makes the Find toolbar go away and gives you a dialog box instead. I didn’t like it.

(I’ll be adding more links and descriptions but don’t want to make people wait… if you don’t see it here, look on the Extensions Mirror or Mozilla Addons)