My favourite extensions for Firefox:
Adblock: the Firefox essential - only see the ads you want.
Greatly increases the speed of browsing over dial-up
Clusty Toolbar: Very handy addition to searching that hides away neatly in the corner of the Navigation Toolbar. Instant encylopaedia and dictionary clips. Try it to see how well it works.
Forecastfox: Weather for anywhere snuggled into your statusbar.
Gmail notifier: Lets you know when you have new mail in your Gmail account - I love Gmail
Magpie: Tool for gathering images. A must for anyone who collects images from the web. So handy you have to install it to believe it!
Quicknote: Notes, quick.
And its all free!