Auto Copy -  Select text and it's automaticaly copied to the clipboard.

Dictionary Search  - Looks up a user selected word in an online dictionary

Disable Targets for Downloads -  Prevents sites spawning blank windows when clicking binary downloads.

Forcast Fox -  Get international weather forecasts from weather.com, and display it in any toolbar or statusbar with this highly customizable extension.

Focus Last Selected Tab -  Brings focus to the last selected tab when closing the active tab.

Session Saver -  SessionSaver will auto-track and restore your browser -exactly- as you left it -- every startup, every time. Not even a crash can phase it: window-positions, tab-histories, cookies + scrolling -- all saved. It has a host of expert options, including session-storage, which allow a high degree of flexibility. Session-removal: right or shift-click in the menu. 'Expert' prefs: click "Simple" in the settings-pane. Suppresses the "warn when closing multiple-tabs" for File > Exit. Hides the file-menu items by default.

GoogleBar -  An unofficial Google toolbar for Firefox. You must uninstall any older versions before installing this.

Adblock -  Once installed, it's a snap to filter elements at their source-address.
Just right-click: Adblock: done. Filters use either the wildcard character (*)
or full Regular Expression syntax. Hit the status-element and see what has or
hasn't been blocked.

AutoForm -  A tool for automatic form filling

Colorzilla -  Advanced Eyedropper, ColorPicker, Page Zoomer and other colorful goodies. With ColorZilla you can get a color reading from any point in your browser, quickly adjust this color and paste it into another program. You can Zoom the page you are viewing and measure distances between any two points on the page. The built-in palette browser allows choosing colors from pre-defined color sets and saving the most used colors in custom palettes. DOM spying features allow getting various information about DOM elements quickly and easily. And there's more...

EditCSS -  Stylesheet modifier in the Sidebar.

Web Developer -  Adds a menu and a toolbar to the browser with various web developer tools. Current tools include converting form POSTs to GETs, hiding images, outlining block level elements, and disabling styles - plus many more.

Bookmark Syncronizer -  Bookmarks Synchronizer is a Mozilla Firefox extension that let you connect to an FTP/WebDAV server and synchronize your bookmarks that are stored in an XML file. Setup is easy; just write in your FTP/WebDAV server address, username, password and a name for the XML file