Without reading the whole thread to make sure I'm not duplicating (lol) I will instead list the couple of extensions that I use.
AdBlock Plus ... can't live without it.
ColorZilla www.colorzilla.com <-- A color picker plugin. When you need to know EXACTLY what color blue that is...
FireBug -- Since I'm a web developer, firebug is GREAT to be able to look at what CSS styles are applied, etc.