Firefox is the best browser out there and it is made even more so by the extensions - all of which are platform independent and so I use these on Windows, OS-X and Linux. Out of the box Firefox outperforms both IE and Safari, but it can be made better!
- Adblock - use the vanilla one (rather than AdBlock Plus) and stop being annoyed by all those ads and banners on the internet that often take longer to download than everything else on the page.
- Adblock Filterset.G Updater - This is a companion extension to Adblock or Adblock Plus and should be used in conjunction with it. This extension automatically downloads the latest version of Filterset.G every 4-7 days. Filterset.G is an excellent set of filters maintained on a daily basis.
- Colourful Tabs - The most beautiful yet the simplest add-on that makes a strong colorful appeal. Colors every tab in a different color and makes them easy to distinguish while beautifying the overall appearance of the interface.
- Daily Dilbert - Adds little icon to statusbar that popup "Dilbert's strip of the day" if clicked. Every geek needs this!
- ForecastFox - Get international weather forecasts from AccuWeather.com, and display it in any toolbar or statusbar with this highly customizable and unobtrusive.
- Google Browser Sync - Google Browser Sync for Firefox is an extension that continuously synchronizes your browser settings – including bookmarks, history, persistent cookies, and saved passwords – across your computers. It also allows you to restore open tabs and windows across different machines and browser sessions. This is the reason I only use Firefox.
- Stop! Hammertime - Stop pages loading with the vocal contributions of MC Hammer.