FireBug is an excellent tool, that is not available for Chrome yet (at least, not with all its bells and whistles, lasts I checked). That's the single reason I still have Firefox installed.
I have found that the Chrome Web Developer Tools can do anything the Firebug can do and usually Chrome does it in a better way in my opinion.
Yeah... I also use the "Web Developer Toolbar" for Firefox. The guy that wrote that released it for Chrome but it doesn't have even half the features as the Firefox version.
And FireBug add-ons, like FlashConsole, make FF indispensable for me.