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.

Comments

[Kiteman]

The FF spellchecker software is a must.

[darkhotv]

Feedly
App tab
Download Helper
and of course AdBlock!

[benny3521]

Stumble Upon
Personas
Xmarks
And if you do social networking and are constantly adding links to web pages on your profile, Addthis is good

[BobS]

Noscript, adblock! Have not had a virus since years!

[orksecurity]

iMacros.

[RavingMadStudios]

I'm told that many people like this little enhancement:

http://www.instructables.com/id/Block-Knex-InstructablesForum-TopicsGroups!-Now/

[geekvig]

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
Flashblock
Greasemonkey
Greasefire - when used w/ Greasemonkey lets you easily use pre-made scripts
gTranslate
Mr. Tech Toolkit - lets you override max version compatibility issues
QuickJava - lets you stop various scripts from running with the click of a button.
StumbleUpon
WikiLook

[steveastrouk]

mousegestures
noscripts
Xmarks,
Downthemall

[frollard]

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

and the 'speed up firefox' tweaks - pipelining and maxrequests.

[Re-design]

Adblock!
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.

[tragicallyhip]

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 .