I use tons of add ons for firefox, too many to describe them all really....

adblock plus-automatically blocks banners and pop ups from sites, as well as being able to be set to block them.
babelfish- automatic translator, set what language you want translated into another language and highlight it/right click
bbcode- adds different BBCode, HTML, XHTML & Symbol options for formatting to the context menu for forums.
better youtube- Several scripts for the greasemonkey add on which adds a few options for playing and searching  on youtube
better privacy- blocks and/or removes long term non-deletable cookies
biofox
compact menu- combines your menu bar into a single selectable icon/button
custom buttons- large selection of custom icons to be used as buttons instead of the standard menu bar
download helper- makes downloading flash/embedded videos from various sites possible
embedded objects 1.9- download embedded objects from sites(inside embed, applet, object and bgsound tags).
firebug-Firebug integrates with Firefox to put a wealth of development tools at your fingertips while you browse. You can edit, debug, and monitor CSS, HTML, and JavaScript live in any web page..
fireshot- screenshot/upload/edit anything on your visible screen
fission- Fission combines address bar and progress bar (Safari style).
flashgot-Download all the links, movies and audio clips of a page
foxy proxy-  FoxyProxy is an advanced proxy management tool that completely replaces Firefox's limited proxying capabilities
ghostery- alerts you when you visit a web site that has web bugs/hidden scripts on it.
ghostfox- Ghostfox stylizes Firefox into Ghostzilla (The Invisible Browser) style. Move your mouse away and Firefox disappears.
google enhancer- adds icons, highlighting, numbering and search options to google
greasefire- notifies you of what scripts are available for greasemonkey
greasemonkey- Allows you to customize the way a webpage displays using small bits of JavaScript
iMacros- you can program multi step repetitive actions into this add on and have it do it for you at a press of a button
image fetcher- adds a save button to images when you hover the mouse over them
informenter-  adds a button next to fields where you enter screenames/passwords/etc and allows you to store the info for easy access
java quickstarter
Linkification- transforms urls into links automatically
media converter- when saving form a webpage, allows you to choose the output format
net framework assistant
multiple tab handler- adds options and capability when managing multiple tabs
new tab button position- small button next to open tabs which automatically opens another
noscript- blocks any active content on a website unless u allow it
personal menu- similar to compact menu(above)
show ip- shows the IP of whatever page youre viewing
skip screen- Skips unnecessary pages on sites like Rapidshare, Megaupload, zShare, Mediafire, and more
stealther- disables cookies, cache, browsing history, download history, saved form info, etc
stylish- change/manage user styles of your browser
tab mix plus- Tab Mix Plus enhances Firefox's tab browsing capabilities. It includes such features as duplicating tabs, controlling tab focus, tab clicking options, undo closed tabs and windows, plus much more
tab sidebar- keeps tabs in a sidebar instead of top of broswer window
thumbnail expander- Expands image thumbnails into full images and embeds some video links
toolbar buttons- more buttons to be used instead of typical menu bar
ubiquity-  very interesting add on im just getting to know...allows you to expand firefoxs ability where instead of just telling it where u want it to go(by typing in a url for example) you can tell it what u want it to do. (such as check weather, check facebook, etc etc)  im still learning about this one
unplug- another tool for saving embedded objects from websites
Web of trust- huge database of websites and how theyre rated on security and safety
Xmarks- bookmarking add on, very useful and saves ur bookmarks/passwords for retrieval later

Just thought Id post this in here since its relevant and not worthy of its own thread.

Firefox 3.5 came out recently and Ive been using it for about a week. Not too many huge changes in general functioning or looks, but it has stopped crashing on me. I was getting one or two crashes per day on a variety of sites and it was getting to be a pain in the ass. Whatever the problem was, they seem to have fixed it.

In regard to add ons, several of mine didnt work right away, but it does seem as though theyre being updated accordingly.  Overall a decent upgrade, especially compared to when they first switched to 3.0.