Chrome comes with the element inspector built in which replicates a lot of the functionality of Firebug.
It has a great DOM inspector (allows editing), network/script/rendering timeline, step-through debugger, profiler, cookie and local database inspector, and Chrome even has a heap analysis tool (see which objects are allocated).
It's far better than Firebug, at least for JavaScript developers, IMO.
Editing style rules applied to elements: yes, you can do this. Select an element in the inspector, and you can click to edit its style rules.