I use Firebug and Web Developer Firefox extensions. One feature I am looking for and which I am not sure if they exist in these extensions is when I am looking at a webpage source, I want to click on a class or id name and somewhere it displays the definition of that class or id. Not the css inheritance hierarchy. Just the particular class or id if they exist

For example:

.....
I click on "header" and I get the css definition or it tells me there are none. I want to filter the css tree hierarchy to just display that class.

Any extensions can do that?