YES, YES, YES!! Finally, a working replacement for the various extensions of this type that have come and gone, especially "Ratings Preview for YouTube" which was sold in June 2018 to a questionable owner (and which stopped working for me recently anyway). I am so happy that someone has stepped up to write a new one and I hope you continue to maintain this and keep it working. A Chrome version is available too (https://chrome.google.com/webstore/detail/youtube-thumbnail-rating/cmlddjbnoehmihdmfhaacemlpgfbpoeb)

Thank you Elliot! As they say, "You the real MVP".

UPDATE: I've encountered a new problem. I can follow the steps to generate an API key, but I can't enter the key into the extension's settings page. The settings pane appears blank. Any ideas what could be causing this?