Which Add-ons or Extensions do you feel are a must have for Firefox?

I have installed Webmail Notifier and NoScript on my Firefox browser and wouldn't be without them.  Are there any you feel are must-have additions to your browser?

26, May, 2010 
It was difficult trying to choose a Best answer because I asked a question that was worded in such a way that every answer was promulgated on the opinion of the answerer. But the robot is bugging me to pick one, so I am going to give to Kiteman for his spell check answer simply because I believe there are a lot of people who could make use of that one.

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Kiteman7 years ago
The FF spellchecker software is a must.
darkhotv7 years ago
App tab
Download Helper
and of course AdBlock!
benny35217 years ago
Stumble Upon
And if you do social networking and are constantly adding links to web pages on your profile, Addthis is good
BobS7 years ago
Noscript, adblock! Have not had a virus since years!
orksecurity7 years ago
I'm told that many people like this little enhancement:

geekvig7 years ago
It depends on what you use the internet for, but these are my favorites:

Adblock Plus
Coral IE Tab - lets you open IE tabs in Firefox window
Greasefire - when used w/ Greasemonkey lets you easily use pre-made scripts
Mr. Tech Toolkit - lets you override max version compatibility issues
QuickJava - lets you stop various scripts from running with the click of a button.

frollard7 years ago
Adblock plus
Flashgot (getting lots/all images/files from a given webspace and more)

and the 'speed up firefox' tweaks - pipelining and maxrequests.
Re-design7 years ago
Download helper!

Just plain firefox covers most of everything else.

Fasterfox finally started giving me trouble so I got rid of it.  I liked it when it was running right though.
Fasterfox will let you scroll down through to the next page of a web site without having to stop and hit NEXT button.
AdBlock...say no more.
LastPass for autologin of username and password,great for all your login needs .