Firebug is much better.

- Tools like YSlow and PageSpeed require Firebug.

- You can't disable the browser cache from FF devtools, which really sucks when you are trying to debug a broken redirect.

- When inspecting an element in FF devtools, :before and :after CSS properties will not be shown. I wasted many hours debugging a CSS refactor because of this (imagine going up the DOM, comparing computed CSS properties until you reach the root). Both Firebug and Chrome devtools will show :before and :after properties.

I think Firebug and the Chrome devtools are comparable in quality.