I actually just opened it to try and remember what annoyed me. It's really the UX which I just don't think is as good: feature-wise it's pretty similar.

(a) It's a lot more cluttered, (b) the UI components which are used are cumbersome: for example the right panel on firebug has a tab at the top while the alternative in chrome isn't as space conscious, (c) icons everywhere which just waste space, (d) I am not sure how to create new CSS properties to attach attributes to, (e) options which aren't obvious to me "Break on sub-tree modifications" -- what will this do?

I could go on. It doesn't feel as responsive as firebug and ultimately firebug just helps me do the most basic tasks as quickly as possible. When I use Chrome I feel confused by the UI, it's difficult to do certain basic things and there are extra features which I don't understand.