Foxmarks
If you use more than one computer then Foxmarks might become handy as it keeps your bookmarks and passwords (optionally) synchronized.
Adblock Plus
This is a must-have add-on for Firefox. Many sites have annoying flash ads in them. Not anymore - Adblock plus blocks them all!
Sxipper
Tired of filling in various web forms and all the spam you get on your e-mail? This add-on automatically fills in registration forms and creates fake e-mails which forward to your e-mail. If you get too much spam from one of the fake e-mail you just cancel forwarding. Simple as that!
Greasemonkey
If you want to make your browsing more comfortable then this comes handy as you can use all the user made scripts. The largest script database is at
Userscripts.
FireFTP
Don’t want to install an extra program for FTP? Download this add-on and you gain FTP access to servers from your browser.
ChatZilla
Another useful add-on with which you can chat on IRC networks and you don’t have to install a separate program!
URL Fixer
Ever mistyped the end of a address? This fixes all of your mistakes - if you type xptricks.ney it automatically corrects it to xptricks.net!
