There are so many firefox extensions out there already, and every time I think to look at the list there are 100s more, many of which sound pretty cool.

I use (on this PC at work)

BugMeNot - allows me to automagically login to sites like New York Times using a login someone else made and submitted to the BugMeNot database

Copy Plain Text - lets me copy text from a web page without the formatting

Linkchecker - checks a page and highlights bad links

Copy Image - lets me copy images directly to the clipboard

Antipagination - takes news storiies that are broken up into multiple pages and smooshes them all into one page

Adblock - blocks ads

Adblock Updater - downloads new ad profiles automatically. I rarely see ads.

Greasemonkey - a multi-extension which you can add various scripts into to do different things - awesome.

Google Send To Phone - send a text message from the browser. even lets you highlight text and send that

Blogger Web Comments - lets you see what bloggers are saying about a page

DownThemAll! - quick downloads of groups of files

I also have a few web nerd specific extensions:

DOM Inspector

Aardvark - lets you remove elements from a page and see what it would look like, as well as other things (this doesn't work with the newest firefox yet)

HTML validator - what it sounds like

Lynx Viewer - shows what a page would look like in Lynx (or to screen reader software)

You can find all of these at this page. Which ones do you use?