FireRainbow
FireRainbow is an add-on for Firefox and Firebug that adds themable syntax highlighting. It uses the JavaScript library CodeMirror to perform the highlighting, and has CSS themes. You can get the themes off 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.