Assume that I\'ve fired up Firefox with Firebug enabled on a random site (say slashdot.org). After loading the site I start editing the page using the Firebug > HTM
Try cssUpdater, it consists of a FireFox add on that listens to your FireBug css changes over a socket server and let you sync your original css source files. You need to install the Desktop application as well.
So a workflow would be:
You can test and use it here, http://www.cssupdater.com