Ask web developers to name their desert island Firefox extensions (ignoring for now the improbability of having a laptop while stranded), and they're bound to put Firebug at or near the top of their lists.
The extension's slogan is "web development evolved," which is apt, given how much it changes one's approach to web work. The tools available in the standard installation of Firebug are super. Coupled with additional functionality available via Firebug extensions (sort of meta extensions, which add on to Firebug itself), you may feel like you have evolved. Build better pages with Firebug.