YouTubeRedux
Browser extension that focuses on replicating old YouTube layout within Polymer design (modern layout).
Chrome
https://chrome.google.com/webstore/detail/youtube-redux/mdgdgieddpndgjlmeblhjgljejejkikf
Firefox
https://addons.mozilla.org/en-US/firefox/addon/youtube-redux/
Deprecated manifest V2 versions can be downloaded from the release page.