1. DownloadThemAll: A very nice download manager with resume and massive download support.
2. Adblock plus: Ad blocker that makes pages load much faster by filtering ads from any websites you visit, you can also add more entries to its list of blocked items. Enjoy Surfing without ads!
3. Mouse Gestures Redox: Do everything using the mouse! Go back, go forward, open a new tab, close a tab, move between tabs, zoom in and out & a lot more!
4. Xmarks: Sync your bookmarks and even your passwords across different browsers, you will no more lose your bookmarks! (Note: This addon was previously named Foxmarks)
5. Foxyproxy: Easily switch between different proxies via profiles and rules, I prefer the basic version as it contains just exactly what I need

Another cool add-on – but I can live without it 🙂 – is the cooliris add-on that allows you to explore and preview thousands of photos in a very attractive way.

All of the above addons are free! Enjoy!