FireRainbow is an add-on for
Firefox and Firebug that adds themable syntax highlighting. It uses the
to perform the highlighting, and has CSS themes. You can get the themes
firerainbow/themes on GitHub and paste them into the Colors tab within Firebug.
The performance is good, and it makes Firebug that little bit more
usable and customised.
Another useful Firebug plugin is
Eventbug. I also wrote a post about Eventbug.