A bit different from other add-ons listed, but Recipefox is absolutely essential for Firefox users who collect recipes using MasterCook or Living Cookbook. Find it at https://addons.mozilla.org/en-US/firefox/addon/6061

Because Salon and Adblock don't get along.

I had to disable it or the pages wouldn't load.

It worked a couple of weeks ago, but now is "Hung" on really big letters everywhere.

D'oh! The most essential.

Blocks flash, but easily lets you run any particular thing you want. You will be shocked at how much flash crap is on every single page. You will be amazed at how much the internet speeds up. After adblockers, flashblocker is the single most valuable add-on to my mind.

IE Tab is another handy add-on, allowing use of FireFox for various sites that don't accept FF... e.g. NetFlix Instant View pages. (It also works for MS Updates)

The Print/Print Preview add-on link is incorrect. It should be https://addons.mozilla.org/en-US/firefox/addon/1778

The add-on I'm most attached to is NoScript, which blocks any flash/javascript scripts from playing. A small 'S' with a stroke through it appears in the bottom right of the task bar, and you can click on it to allow sites, temporarily or permanently. I've converted my wife and daughter to it as well.

Remove It Permanently lets you not only block ads but collapse the space where they resided. It is configurable to specific remove ads on specific pages, to remove ads on entire domains, and in other nifty ways.

I can't believe NoScript isn't #1 on your list, it's absolutely essential. I would not use Firefox without it. It's your best protection against cross-site scripting attacks, and just generally makes the web a safer place to browse.

Another useful add-on is Greasemonkey. I use a nice script that hides a lot of the junk on the Facebook hompage.