Firebug was amazing and one of the reasons I started doing front-end development again, after swearing it off because IE 5.x made it such a frustrating experience.
It flat-out made training people on CSS tolerable. I could have them install the Bookmarklet (before it became an Extension), show them how to use it, and boom, instant productivity boost. Being able to dig through the DOM and figure out what element was applying excessive margin or padding reduced 80% of my "heeeelp!" requests.
Within months I couldn't imagine working without it.