Quoted from the Firebug FAQ:
Editing Pages
Can I save to the source the changes I made to the webpage I'm seeing?
Right now you can't. As John J. Barton wrote on the newsgroup:
Editing in Firebug is kinda like taking out the pickles from and adding mustard to a restaurant sandwich: you can enjoy the result, but the next customer at the restaurant will still get pickles and no mustard.
This is a long-requested functionality, so someday it'll be available directly from Firebug. Meanwhile, you can try Firediff, an extension for firebug by Kevin Decker.
How can I output all changes that have been made to a site's CSS within firebug?
That's a feature implemented in Kevin Decker's Firediff.
