Here are the 5 best addons that I find useful. These may not be what everyone thinks. Its just what I use.

1. Remember The Milk Addon for Gmail - This addon gives users the usability of both Gmail and RTM (Remember The Milk). All you have to do is go onto Gmail, and you see all your RTM Lists side by side. A must have for users of Gmail and RTM.
2. Google Redesigned - If you use Gmail and Google Calendar. Give yours a new look. It looks very nice. Try it for yourself.
3. Google Gears - This is becoming a must have, like Adobe Flash. This, makes it easy for you to use web applications while your offline. Great for when you are travelling. Only a few websites work with this, but these include RTM, Google Reader. There are rumours that Gmail and Google Calendar are also getting this.
4. Foxmarks - Do you use Firefox on multiple machines? If you do, you will have trouble keeping your bookmarks on all the machines. A easy way to keep them synchronised is using Foxmarks. Even if you only use one computer, you can use Foxmarks as a backup. Also, if you are using a public PC, e.g. at a Internet cafe, you can login to the site, and see your bookmarks, edit them, and add more. And whey you start Firefox at home, all you new/edited bookmarks will be changed, just like you did on the website.
5. ForecastFox - If you don’t get away from your computer allot. But want to see what the weather is like, or going to be like in the next few days. Then ForecastFox is a great extension. You can see upcoming weather, and see a radar view of the weather. Its very good.

One specifically for windows vista

Glasser - If you like how Internet Explorer gives you the Aero effect. But, miss it in Firefox. Well if you use this extension, then you can get the same aero effect.

So, if you use Firefox, What extensions do you use.

Thanks,

Danny