Here’s a list of the major firefox addons that I use

LastPass

This robust password manager is a must-use freeware tool that supports multiple operating systems and browsers. With LastPass, you create a master password, and then it uses automatic form filling and one-click log-in to simplify password entry. LastPass will create secure passwords for you, as well as allow for secure password sharing, password import and export, secure note-taking, and password backup and restore.
Download LastPass addon here

Delicious

Here you got one very usefull tool. It helps you in everything related to bookmarks; you can add it to almost major browsers, and also access it on the web. You can share your bookmarks with your friends and even make them private. This is even better, as you can add a lot of information regarding the bookmark, using it as a micro blog.
Defenitly better then any other bookmark aplication.
Download Delicious addon here

NoScript

This free Firefox extension blocks JavaScript from running without your permission, but NoScript doesn’t allow you to stop its own devices. After installation, NoScript adds only a small blue button marked “S” above the toolbar. To configure the program options, simply click the button to display a very conventional interface.
Download NoScript addon here

Firebug

Have you tried out Firebug yet? If not, you might want to consider giving it a go. It’s one of a few Firefox extensions that I use almost daily in my web development work, and I can guarantee that it’s made my life at least a little bit easier. I most often use Firebug for its ability to “inspect” various elements on the page.
Download Firebug addon here

Better Privacy

Better Privacy (free) can get rid of privacy-busting Flash cookies, also known as local shared objects or LSOs, which are used by many sites to supplement the normal HTTP cookies. Flash cookies can be used for similar things as HTTP cookies, including saving preferences and tracking where you surf, but you can’t delete Flash cookies using basic browser options like you can with regular cookies.
Download Better Privacy addon here

Aardvark

Cool utility for cleaning up a page (for printing or better viewing) as well as for web development. Displays a red rectangle on the element under the mouse — hit “w” to go wider (get the containing element), then hit other keystrokes to remove, etc. Powerful and user-friendly selector utility for selecting elements and doing various actions on them.

Download Aardvark addon here

AdBlock +

Annoyed by adverts? Troubled by tracking? Here’s a very good addon that everyone should use to avoid those unnwanted worms. It’s very easy to use. You simply have to click on the link or image you want to block and add it to the blacklist. It wont show anymore. A very usefull tool for every developer or any home user should use on a day to day basis.
Download Adblock+ addon here