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.

Thank you for Firebug. It's really good. My only complaint about Colorzilla is that it's somewhat iffy on words and along the edges of borders. Then again, that's what Firebug's CSS feature can help with.