I am the author of CSS-X-Fire which Sorin Sbarnea also kindly posted about in this thread. Guess I'm a bit late ;)
CSS-X-Fire emits CSS property changes from Firebug to the IDE where the changes can be applied or discarded.
There are a couple of advantages with this solution over most of the other existing tools which only know know about the filenames and the content downloaded by the browser (see NickFitz comment in the original post).
Scenario 1: You have a website (project) which has a handful of themes from which the user can select from. Each theme has its own CSS file but only one is known to Firebug, the current one. CSS-X-Fire will detect all matching selectors in the project and let you decide which should be modified.
Scenario 2: The web project has stylesheets created compile-time or during deployment. They might be merged from several files and the file names may change. CSS-X-Fire doesn't care of the names of the files, it only deals with CSS selector names and their properties.
Above are examples of scenarios where CSS-X-Fire excels. Since it works with the source files, and knows about the language structure, it also helps to find duplicates not known to Firebug, jump-to-code, etcetera.
CSS-X-Fire is open source under the Apache 2 license.
Project home: http://code.google.com/p/css-x-fire/