Favorite Firefox plugins/add-ons?

OK, this isn’t a plugin. But this is the cutest theme ever. EVAR!
Foxkeh Firefox Theme

As for plugins, I only have de.licio.us and StumbleUpon addons installed.

If you run Firefox on Windows, NoScript is a must.

Another vote for NoScript.. other than that i don’t really mess with any others.

Essential:
AdBlock Plus
Greasemonkey

Timesaver:
Ubiquity

Prettifier (in a useful way):
Fission

Useful function:
Read it Later (an easy “to read” list)
Tidy Browser Button (keeps my screen clean—I like having my window the “right” size, and yes, I have many OCD tendencies)

Stylish – great way to modify any webpage
GreaseMonkey – again, modify/adapt pages use frequent
AdBlock Plus – ‘nuf said
Download ‘Em All – downloads all the links on a page (great for use with Google Docs)
Ubiquity – awesome. freaking awesome. (i wrote my own hype machine script!)
TabFX
gDocsBar
ForecaseFox
Read It Later

I use de.licio.us, Gmail Manager and Foxmarks.

a lot, but the only one that matters: https://addons.mozilla.org/en-US/firefox/addon/5846

Just added Read It Later. Thanks! GQ.

AdBlock Plus “Ads were yesterday!”
AdBlock Plus:Element Hider (an add-on for an add-on, spiffy huh?)
WebMail Notifier Notification of new email in any of your webmail accounts (gmail, yahoo, ..)
GreaseMonkey Custom scripting to manipulate webpage elements
Stylish Custom scripting to manipulate webpage look/scheme
Foxmarks Synchronize bookmarks (& optionally passwords) between different computers
Google Bar (but I move the buttons off and hide the bar)
Stumble Upon
Facebook Toolbar (again, move the buttons off, but it always seems to break and stop notifying me of updates and I can’t find a way to fix it)

I play with a lot of add-ins. Thank you for sharing!

Everyone has said all the things I use to modify Firefox, so I’ll list some of the things I use to extend its functionality.

-Play NES games with FireNES!
-Turn it into an IRC client with Chatzilla!
-Aggregate/Read RSS and Atom feeds with Sage!
-Turn it into a BitTorrent client with Fire Torrent!

And this one’s solely cosmetic, but I really, really dig the Chromifox theme.

Key Scrambler, an absolute necessity for peace of mind.

My two favorites are definitely the Stumble Upon bar and Forecast Fox.

Download Statusbar
CoolIris
Fast Dial
Firefox PDF Plugin for Mac OS X
Foxmarks
Ubiquity

@benseven How could I forget Download Statusbar? I love that one.

Can’t Live without:
NoScript
Foxmarks
Smart Bookmarks Toolbar
Gmail
FireFTP

Very helpful:
Adblockplus
Sharethis
Stumbleupon
Colorzilla

Newly in love with:
GoogleTube

Helpful but not used frequently
Cooliris (very awesome but I forgot to use it!)
PayPerPost toolbar
Orbit Downloader
Scribefire

CyberSearch – search google, wikipedia, etc. from the location bar.

Locationbar^2 – linkifies URLs in the location bar

Indispensable:
AdBlock Plus
Blur (was Do Not Track Me up until this past week)
Chrome’s ePub book reader
Tab Mix Plus (a wealth of options for personalizing my collection of pinned App Tabs)
Adobe PDF plug in for Firefox

Quite handy:
Word Count Tools
the Amazon button

Also nice:
Flag fox (shows the national flag of the country where the server a website comes from is located)
the Green Frog theme is truly beautiful