Stephen Oney @ Carnegie Mellon
http://stephenoney.com/firecrystal.php

FireCrystal is a Firefox extension that helps designers and programmers alike figure out how interactive behaviors on the web work. FireCrystal allows users to "rewind" their interactions with web pages while showing the relevant code.

track DOM changes

@igrigorik: "nifty FF extension for replaying / rewinding DOM changes: http://bit.ly/2aGyPH" (from http://twitter.com/igrigorik/status/3839646962)