I know this doesn't answer your question, but surprisingly, Internet Explorer 8's Firebug clone "developer toolbar" (accessible via F12) offers the option to "save html". This function saves the current DOM to a local file, which means that if you edit the DOM somehow, e.g. by adding a style attribute somewhere, this will be saved too.
Not particularly useful if you're using Firebug to mess around with CSS like everyone does, but a step in the right direction.