Firebug works on the computed CSS (the one which you get by taking the CSS in the files and applying inheritance, etc. plus the changes made with JavaScript). This means that probably you couldn't use it directly to include in an HTML file, which is browser/version specific (unless you care only about Firefox). On the other hand, it keeps track of what is original and what is computed... I think it should not be very difficult to add some JS to Firebug to be able to export that CSS to a text file.
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