These are the Firefox 3 plugins I cannot live without:
Foxmarks Bookmark Synchronizer
If you use Firefox on more than one computer, you'll want Foxmarks. Install Foxmarks on each computer, and it will work silently in the background to keep your bookmarks synchronized. You can also log in to my.foxmarks.com to manage your bookmarks from any computer.
Custom Download Manager
This addon allows you to personalize your download manager. Now, you can open the download manager on a new tab or on the sidebar.
Open Tabs Relative
Tabs Open Relative makes all new tabs open to the right of the current tab, rather than at the far right of the tab bar.
Open Image In New Tab
Adds right-click context menu item for opening images in new tabs.
Undo Closed Tab Button
Tired of going to History -> Recently Closed Tabs just to undo a closed tab? Then this extension is for you! This extension allows you to undo closed tabs via a toolbar and/or tab bar button or the right-click context menu.
Ever been annoyed by all those ads and banners on the internet that often take longer to download than everything else on the page? Install Adblock Plus now and get rid of them.
Adblock Plus: Element Hiding Helper
Fight the text ads! Element Hiding Helper is a companion extension for Adblock Plus meant to make creating element hiding rules easier.
Flashblock is an extension for the Mozilla, Firefox, and Netscape browsers that takes a pessimistic approach to dealing with Macromedia Flash content on a webpage and blocks ALL Flash content from loading. It then leaves placeholders on the webpage that allow you to click to download and then view the Flash content.
Mouse Gestures Redox
Allows you to execute common commands (like page forward/backward, close tab, new tab) by mouse gestures drawn over the current webpage, without reaching for the toolbar or the keyboard.
I'd love to know what plugins you use, leave a comment with a description.