Kevin Decker has released FireDiff a very cool Firebug extension that tracks changes to the page and CSS.
Firediff implements a change monitor that records all of the changes made by firebug and the application itself to CSS and the DOM. This provides insight into the functionality of the application as well as provide a record of the changes that were required to debug and tweak the page’s display.
The release, which you can find here uses recent features of the latest Firebug 1.4 code. Early days!