• Adblock Plus – Ads were yesterday!
• Browser View Plus – View Firefox pages in IE or other external browser
• Close Button – Adds a Close Tab button to the toolbar
• Cooliris – Cooliris transforms your browser into a full-screen 3D Wall for searching, viewing and sharing the Web.
• Download Statusbar – View and manage downloads from a tidy statusbar
• DownThemAll! – The mass downloader for Firefox.
• English (Australian) Dictionary – I’m sick of all my favoUrite coloUrful language being marked incorrect.
• Extension List Dumper – Dumps a list of the installed extensions.
• Favicon Picker 3 – Replace bookmark icons from the bookmark properties dialog.
• FaviconizeTab – The width of the specified tab becomes small up to the size of favicon.
• FEBE – Backup your Firefox data
• Fetch Text URL – Open text URLs from the context menu.
• Firebug – Web Development Evolved.
• FireFTP – FTP Client for Mozilla Firefox.
• Flashblock – Replaces Flash objects with a button you can click to view them.
• Google Gears – These are the gears that power the tubes!
• IE View – (Disabled) – Open pages in IE via Firefox menus
• Microsoft .NET Framework Assistant – Adds ClickOnce support and the ability to report installed .NET Framework versions to the web server.
• NewTabURL – (Disabled) – Select default URL when new tabs are opened.
• nURL Suffix – Change the prefixes and suffixes which are used to complete a URL.
• OpenDownload – Extends the “Save” dialog by a button to open the file directly.
• Pearl Crescent Page Saver Basic – Save an image of a web page to a file.
• Prism for Firefox – Create Prism applications directly in Firefox
• QuickRestart – Adds a “Restart Firefox” item to the “File” menu.
• Smart Bookmarks Bar – Hides bookmarks’ names in the bookmarks bar.
• SQLite Manager – Manage any SQLite database on your computer
• Tab Mix Plus – Tab browsing with an added boost.
• Tab Scope – Preview and navigate tab contents through popup.
• Weave – (Disabled) – Weave is the Mozilla Labs prototype for online services. (Well, it would be if it worked, which it doesn’t).
• Web Developer – Adds a menu and a toolbar with various web developer tools.
• WebAii 2.0 Automation Infrastructure – Firefox Client – WebAii’s Firefox support client.
• Xmarks – Bookmark Sync and Web Discovery

That’s just my list or part thereof that I’m using today. There’s plenty more out there and it is well worth exploring the gamut.

[edit - Sep 29 .09] Check these out too – http://designreviver.com/freebies/22-firefox-3-plugins-web-designers-cant-live-without/