Firebug is an extremely useful addon, it basically eliminates the use of alot of other addons that eventually completed the same task. Examples such as Venkman, View Rendered Source Chart, Console2, Aardvark, and MochiKit JavaScript Interpreter.

Although, it does lack a lot of features that those specific addons provide. So it mostly just depends on the specific task at hand.

Plus Chrome is ftw. Just sayin'