The things you edit in Firebug are just Firefox's interpretation of
the web page so there can be quite a few differences between what you
see in Firebug and what is actually in your pages HTML and CSS. This
makes it very difficult to save live changes in the way that you
describe. Your best option is probably to install a Firebug extension
called FireDiff. FireDiff allows you to keep track of all the changes
you have made to a change and is available from
https://addons.mozilla.org/en-US/firefox/addon/13179