The list:

• del.icio.us – Provides basic integration with del.icio.us so I can update my tags easily from within the browser (no need to navigate to del.icio.us to manage these).
• DOM Inspector – Inspect the DOM of HTML, XUL, and XML pages, including the mail chrome. I use this primarily for tweaking/developing javascript.
• Dummy Lipsum – Use to generate dummy text. This is great when you need to test/view pages as they would appear with realistic content.
• Foxmarks Bookmark Synchronizer – Foxmarks is a free add-on for your Firefox browser that syncs and backs up your bookmarks and passwords across multiple computers and more. Since I have a few computers that I work with, this service makes it easy to keep my bookmarks synchronized.
• LiveHTTPHeaders – Great for viewing HTTP request/response headers. This is a handy utility when doing web development.
• NoScript – Preemptively blocks script execution until you approve, creating a whitelist of trusted web sites from which script execution is allowed. This makes for a safer browsing experience by blocking unknown third party and cross site scripting.
• OPML Support – Adds OPML support for importing/exporting links in the OPML format using the Firefox bookmark manager. I used this a couple times when importing my live bookmarks into Wordpress.
• Sage Too – I think this is a great feed reader for viewing my live bookmarks.
• Semantic Radar – Exposes Semantic Web metadata published on web pages, providing a convenient means for viewing and sharing discovery of the data with the community. I’m using this to get a better understanding how Semantic Web metadata is being published and used.
• TwitterFox – This extension adds a tiny icon on the status bar which notifies you when your friends update their tweets. Also it has a small text input field to update your tweets. Similar to the del.icio.us integration described above, this allows me to keep up with twitter from within my browser without having to navigate directly there.
• User Agent Switcher – The User Agent Switcher extension adds a menu and a toolbar button to switch the user agent of the browser. This is useful to testing (and in some cases circumventing) browser-specific functionality on various sites.
• Web Developer – The Web Developer extension adds a menu and a toolbar to the browser with various web developer tools. I find I use these tools extensively when I have to do UI tweaks…definitely a must have.

Thanks to all of the developers of these add-ons for their generous contributions! They’ve helped to increase my productivity.