Occasionally and unpredictably stops displaying the ratings bars (maybe a YouTube API quota issue per the 0.0.42 release notes?) but this add-on is otherwise very well designed and has no doubt saved me hundreds, if not thousands, of wasted clicks and page loads over the past year or two.
Thanks for reporting this. I just released an update to address the recent issues, but the extension now requires users to enter a personal YouTube Data API key. Instructions for setting up a free YouTube Data API key, and an explanation for why this is required, can be found here: https://github.com/elliotwaite/thumbnail-rating-bar-for-youtube
