Yesterday, I was watching someone on the bus scroll on their iPhone and I was thinking "Why doesn't my scrolling look that smooth on my laptop"? After a quick search I found the "Yet Another Smooth Scrolling" add-on for Firefox and it works perfectly!
I have tried lots of different add-ons, but the ones that I find essential are:
Firefox (in order of importance)
-Foxmarks Bookmark Synchronizer - allows backing up and syncing of bookmarks (and recently passwords) between computers
Adblock Plus - blocks all of those annoying ads
Download StatusBar - makes a smaller download status bar
Yet Another Smooth Scrolling - smooth scrolling
Greasemonkey - allows tons of different of scripts (including some nice ones for browsing Craigslist)
Slideshow - allows easy viewing and resizing attached pictures
Signature Switch - a single button that quickly turns on or off your signature (and allows multiple signatures)
If you have any other essential add-ons, please let me know!