These are some Firefox add-ons that I find useful:
ColorZilla is a Firefox add-on that provides tools for web page color analysis. (As of September 2010, ColorZilla does not work with any other browsers.) Once installed, ColorZilla provides an eyedropper and a pull-down menu in the lower left corner of the browser frame. The eyedropper can be used to find and copy (in various formats) the values for any color on a web page. In addition, the pull-down menu can be used to access a color picker, palette tool, and webpage DOM color analyzer.
Most of the add-ons I described in the original version of this post are no longer available or no longer relevant. For historical purposes, these are described below.
Flashblock was a Firefox add-on that prevented automatic execution of Flash objects. Since Adobe has ended all support for Flash Player (as of December 31, 2020) the best way to prevent execution of Flash objects is to uninstall all Flash Players.
BetterPrivacy was a Firefox add-on that removed files created by Flash to store certain kinds of user data.
Xmarks, first known as Foxmarks, was a browser add-on that synchronizesd bookmarks across browsers and computers. In addition, Xmarks stored the same bookmarks online for web access from any computer. I really liked Xmarks, and I was happy when LastPass took steps acquire Xmarks in 2010 in order to keep it from going under. I was sad when LastPass decided to discontinue Xmarks in 2018.