Add-ons extend Firefox, letting you personalize your browsing experience. They can make a tiny tweak, or bring a whole new range of features. Here are some of my favorites:

• Flashblock - Blocks all flash on webpages. Only loads if you click on it. Great for sites like MySpace that have unwelcome and risky ads.
• Forecastfox - Weather in your lower info bar; current temp and conditions, 2 day forecast, hover over for radar.
• del.icio.us Bookmarks - If you have a del.icio.us account you can post sites with one click.
• Download Statusbar - Instead of a pop up with download status, you get a nice little progress bar in the bottom left of the status bar.
• Google Browser Sync - Syncs your passwords, bookmarks, & more with Firefox on different computers.
• StumbleUpon - tool bar that allows you to randomly find kewl websites with one click.
• Tab Mix Plus - Tab manager for Firefox. Total custom controls.