Safari's Web Inspector (used verbatim in Chrome) is still missing a few features vs. Firebug (mostly with editing, and integration with webapp frameworks), but it's STABLE, and fully integrated.

Firebug crashes constantly, is unusable with many sites, and is never quite in sync with Mozilla's development plans. Mozilla blew it big time promoting their completely useless "Web Developer" extension for years (even shipping it in the installer), and completely ignoring Firebug.