K, I dont use many but here goes...
- The dictionary / spell checker obviously
- Adblock plus - works very well at killing all the ads on sites like this
- Gmail Manager - because gmail is my main email client and the notifier is useful
- IE View lite - When you need to view pages in Internet Explorer this opens them in an IE Tab
- Go To Google - I know that there is a google search box but I like this extension a lot. It sits in the bottom corner and is just a [G] icon that opens a new google tab when you click on it
*** Ones that aren't working in FF3 ****
- New Tab Homepage - Simply loads your homepage (google) when you open a new tab. Rather than a blank page.
That is about all. I have tried loads but ended up uninstalling them to speed things up again.
My favourite tip is Ctrl+T which opens a new tab.