A nice feature of web inspector and of course firebug is that you can manipulate by changing definitions, positions, classes etc etc. So what happens with complex pages is that you have to click through a few nodes and change things here and there until this look just as you like it.
What I really miss with firebug is that I want it to give me a summary or overview of what exactly I changed, call it a diff, so that I don't overlook things when I'm putting my efforts back into my local css. Maybe a panel or a window that I can open that shows me exactly "on this node you set this style, in this class you modified this font" etc etc. That would be SOOOO handy.
Oh, and don't forget: whatever is shown in this summary must be properly copy/pastable :).
Firebug have Firediff now http://www.incaseofstairs.com/firediff/
Would be great to see something like this in Web Inspector.
We will be tracking ability to export CSS in 38578 and an ability export
changed code in general in 20633, so I am marking this as a duplicate, because
it essentially asks to solve the same problem.
*** This bug has been marked as a duplicate of bug 38578 ***