For those of you who use firefox, what extensions do you use? Which do find essential and which are expendable?

Here is my current list:

Adblock
Filters ads from webpages. This make browsing much less annoying, however raises significant ethical issues.
Status: ESSENTIAL

Adblock Filterset.G Updater
Updates Adblock with Filterset.G. This keeps your list of ad-serving websites up to date.
Status: ESSENTIAL

Greasemonkey
A User Script Manager for Firefox. There are thousands of well written scripts that can add needed functionality to many of your favorite sites.
Status: ESSENTIAL

Platypus
Dynamically create Greasemonkey Scripts. Not as cool as it could be.
Status: NONESSENTIAL

All-in-One Gestures
Adds a number of mouse gestures that make mouse only browsing much easier.
Status: ESSENTIAL if use a mouse a lot.

Fasterfox
Performance and Network Tweaks for Firefox
Status: Fun to Have but Ultimately Expendable

WellRounded
Provides Rounded URL and Search Bars. Adds a nice finishing touch to a nice theme with squared boxes.
Status: NONESSENTIAL

MenuX
Very Useful for Full Screen Browsing but a bit of overkill.
Status: Just installed..Review Pending

Tab Mix Plus
Adds Essential Tabbed Browsing Options. Hopefully most of the functionality in this extension will be included in the 2.0 final.
Status: ESSENTIAL until 2.0

BlogThis
Makes it easy to add interesting finds to your blog.
Status: ESSENTIAL (if you use Blogger)

Google Notebook
Lets you add pages and notes to your Google Notebook
Status: Just trying it out.

McAfee SiteAdvisor
Status: Just trying it out.
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If you do choose to use AdBlock, I highly suggest that you whitelist our beloved UGN and look for the upcomming "Is Adblock Ethical?" post in Rants and Debates.

I encourage everyone to post their list of extensions along with their impressions.

Google Toolbar
Well, its googles toolbar, for firefox...

HTML Validator
Allows you to check the validity of html/xhtml pages.

ColorZilla
Allows you to select the color value of any portion of your page via a dropper; also adds a color picker to your browser to aid in adjusting colors.

ReloadEvery
Allows you to set a site (or set of sites) to reload on a set period of time.

Tabbrowser Preferances
Adds various enhancements that effect tabbed browsing.

Dictionary Search
Allows you to add various "searches" to your right click menu, which will allow you to search the definition of words, search for them on google, or any other search (torrents, cracks, whatever you can plug into a url).

Stumble Upon
Allows you to go to "random" sites and view what users have to say about them.

Open Link In
Adds more options to your right click and tabbar menus, allowing you to open links and images in new tabs and more.

Secure Password Generator
Allows you to generate a random password based on several selections (length, letters-capital, letters-lower, punctuation, etc)