Since I've been setting up Firefox and associated apps on my new work laptop, I thought I'd list my current set of favorite add-ons for Firefox 3.
I've used Firefox and some of these add-ons for years, and they help to make Firefox that much more useful and safe for day-to-day browsing. There are literally thousands of add-ons to choose from, so these represent the best and most useful of the ones available in my experience.
Adblock Plus 0.7.5.5: blocks ads, period. Modern replacement for Adblock + Filterset.G Updater.
Download Statusbar 0.9.6.3: makes it easier to track and manage multiple downloads, nicer interface than the download manager included with Firefox 3. Alternately, if you need to queue up and manage dozens or hundreds of downloads, use DownThemAll! 1.0.3 instead.
Video DownloadHelper 3.3: downloads videos from sites like YouTube for local use.
Feed Sidebar 3.1.4: the least annoying feed reader I've ever used. I love the idea of RSS/Atom feeds as a way to view and stay up to date with content, but Feed Sidebar is the first reader I've used in 3 or 4 years that I didn't delete after 2 weeks of use.
NoScript 220.127.116.11: protect your Firefox session from malicious code. Practice safe browsing! Firefox with AdBlock Plus and NoScript installed makes your browsing about as safe as you can get on the web these days.
QuickProxy 2008.08.04: gives you a statusbar button to turn proxy support on and off. Couple this with PuTTY or another SSH client configured for SOCKS5 connection forwarding to your desired SSH server, and you've got an instant VPN for your Firefox session.