It's a chrome/firefox/safari extension used to highlight new comments on a comments page that was visited earlier on reddit. It helps keep track of new comments when you revisit a comments page.
The extension is built using Kango Framework & works on multiple browsers.
$wget http://kangoextensions.com/kango/kango-framework-latest.zip
$unzip kango-framework-latest.zip
$python kango_dir/kango.py build ./Reddit-New-Comments-Highlighter
Chrome: https://chrome.google.com/webstore/detail/reddit-new-comments-highl/ajdilinnnkbmpoegibgacadjlblmpjad
Firefox: https://addons.mozilla.org/en-US/firefox/addon/reddit-new-comments-highlighte/