Other Adaptavist staff regularly tell me I have an insane number of Firefox plugins installed, but I really can't live without them.
I'll start by listing them and will write some more about them if/when time permits...
Adblock Plus
All-in-One Sidebar
Brief
This is by far the nicest RSS feed reader I've found for Firefox. All the others have some annoyance that results in them being uninstalled after a week or less, whereas Brief has stood the test of time.
Dictionary
It surprises me that all browsers don't come with built-in spell checking.
cmSiteNavigation Toolbar
ColorZilla
Colour Contrast Analyser
CSSViewer
Document Map
Download Statusbar
So much nicer than the inbuilt download manager IMHO.
DragNDrop Toolbars
Extended Copy Menu
Extended Status Bar
Fasterfox
I found the more aggressive performance settings made browsing less reliable, however the right-click "Clear Cache"
FAYT
Fixes some annoying usability issues with the in-page Find toolbar.
Find Toolbar Tweaks
Firebug
MUST HAVE!
FireFoxMenuButtons
Did I forget to mention that I've also completely customised the entire navigation in Firefox?
Forecastfox
I never take much note of the weather, but I find it strangely satisfying seeing a weather report in my menu toolbar.
FoxClocks
With clients in 55+ countries, this is essential.
FoxyTunes
Control WinAMP direct from my web browser.
I've also installed the "Finespun" theme for FoxyTunes.
Fullerscreen
Perfect for browser-based presentations when using a projector.
Go Up
You have a back and forward button in the browser - this plugin adds an "Up" button allowing me to quickly jump up the URL path hierarchy.
Googlebar
Most of it is hidden away, I just drag the useful bits on to other toolbars.
Greasemonkey
Allows me to kick other websites in to shape by adding my own scripts and CSS, etc.
Header Monitor
Useful for monitoring specific headers in HTTP(S) requests.
HTML Validator
IE Tab
See what the page looks like in Internet Exploder, directly within Firefox 
Link Alert
I love this plugin - shows a small icon next to the cursor when you hover over a link so you know what sort of thing the link points to. For example it shows a PDF icon when hovering over a link to a .pdf file.
Live HTTP Headers
Live PageRank
Tucked away in my status bar, it shows me the Google page rank of any page I visit 
Locationbar2
This is another dream plugin, it replaces the URL in your location bar with a customisable breadcrumb trail.
MeasureIt
Open Image In New Tab
This should really be a standard feature of the browser IMHO.
PDF Download
Asks you whether to download, view as HTML or PDF when you click a PDF link.
ScrapBook
Occasionally use this to grab snippets of web pages, but will probably disable it soon as I don't use it often enough.
Skype Extension for Firefox
Speed Dial
Allows you to create a grid of bookmark thumbnails to quickly find frequently visited URLs.
Sun Cult
Pointless for me, but probably useful for Pagans and anyone else who needs to know what the Sun and Moon are up to. I just have it because I like seeing the moon phase in my toolbar.
Tab Effect
DirectX rotate effect when moving between tabs. Pointless, sometimes annoying, but I like it anyway.
Tab Mix Plus
MUST HAVE!
Tab Scope
Shows page thumbnail when you hover over a tab.
Tails Export
Greate for spotting and exporting microformatted information within pages.
Talkback
Always let Mozilla Foundation know when your browser chimps so they can fix the bugs and make it better.
Toolbar Buttons
No, really, I have gone insane with customising the navigation in my browser.
Update Notifier
I don't like waiting for a restart to see when there are upgrades for my plugins - this shows a nice little icon when updates are available so I can restart Firefox there and then to get them.
UrlParams
Useful for viewing and editing the URL parameters on HTTP GET (and also POST params as well IIRC).
Web Developer
A must have for any web developer! Herds of very useful features for lots of techie things.
XML Developer Toolbar
Still unsure about this one.
XPather
Very rarely used, but when I do need it, it's a life saver. Click any element on the page to get the full XPATH to that element.
YSlow
Tells you why your web pages load too slowly (used in conjunction with Firebug).
Disabled Plugins
There are a bunch of plugins that I generally leave disabled when I'm not using them (reduces Firefox start-up time)...
ChatZilla
This is disabled most of the time as I don't use IRC much - it's a browser-based IRC client that works surprisingly well.
CLC-FireVox (and related plugins)
Adds text-to-speech features to Firefox, great for testing accessibility requirements. Gets annoying during early development cycles though.
Console2
I'll probably bin this because I have Firebug which is way better.
Facebook Toolbar
I use Facebook a fair bit, but haven't yet had the need to use the toolbar. It'll probably get deleted unless I start making regular use of it.
FireFTP
Fantastic Firefox FTP client, but unfortunately not yet able to deal with some of the security we apply to our FTP servers.
Graphing Calculator Toolbar
Installed because it's geeky and cool, but I hate maths and will never use it.
Image Counter
Sometimes useful, but the Extended Statusbar now shows this info so this will probably get uninstalled soon.
Modify Headers
Palette Grabber
Print Hint
Tab Catalog