ColorZilla is amazing. It's the one best developpment tool that I'd recommend.
Personally I find firebug is just too hard to manage, hasn't grown on me. Either it can't do some things I want it to, or I just don't know how to use it good enough.
JSview: VERY handy if you look at other sites often. I mostly use it for CSS viewing, since it can view both attached JS and attached CSS. Handy, since you'll not need to view source.
CSS viewer is nice too... I have a few dislikes with it (like with font size, it just calculates it in pixels, instead of displaying em or pt values), but it's quite useful if you want to know info on any element (such as margin,padding, and border information on an element, what fonts are used, dimensions, etc.) in your page, good for verification or debugging.