Firefox Addons

royal

Nice to hear what people use and why.

I use:
Google Preview - when doing a search in Google using Firefox, thumbnails appear next to the search to get a preview of what you'll look at when you open the site
Foxmarks - use an online account where you store your bookmarks for synchronization across multiple machines

dynamik

Google Browser Sync is my big one. Syncing everything between multiple machines is amazing. I didn't even really use bookmarks before this because they were such a pain to keep track of.

I haven't used Foxmarks, but it looks like that only does bookmarks, correct? Google's will keep track of your tabs, cookies, etc. Plus I already have a Google account, so I don't need to keep track of another u/p.

User Agent Switcher is nice if you need to appear to be IE.

Showcase and TabScope and neat but nothing amazing.

Web Developer and Greasemonkey can be quite useful.

I'm also learning Japanese now as well, and RikaiChan is a pretty slick Japanese dictionary.

KGhaleon

Text-to-image for converting links on pages.
Chatzilla-IRC client for firefox.

brad-

IE tab - for those sites that need IE, so i can still use FF.
Forecastfox - so i can have the weather on my status bar, with a mouseover radar.

Skin - Redshift2. I like the dark theme.

Ahriakin

ReloadEvery : Refreshes my HTTP console windows automatically, keeps the info up to date and stops them timing out.

Firecat maintains a list of good security add-ins too though I havent' used them yet, was just reading about it this morning: http://www.security-database.com/toolswatch/FireCAT-Firefox-Catalog-of,302.html

JDMurray

The standard add-ons I use in every FF browser are: IETab, FasterFox, FlashGot, NoScript, and SafeHistory. For a software testing computer, I will also install Greasemonkey and Chickenfoot. I also have a lot of add-ons installed that I rarely use (ChatZilla, VideoDownloader) or just never ended up using.