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.
➜ $?=0 ➤ make update
wget -q http://kangoextensions.com/kango/kango-framework-latest.zip
unzip -q -o -d kango_dir kango-framework-latest.zip
rm kango-framework-latest.zip
>>> 1s elasped...
➜ $?=0 ➤
➜ $?=0 ➤ make build
python kango_dir/kango.py build ./
[ INFO] Contact [email protected] to enable IE support
[ INFO] Running Kango v1.8.0
[ INFO] Building chrome extension...
[ INFO] Building firefox extension...
[ INFO] Building safari extension...
>>> 2s elasped...
➜ $?=0 ➤
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/